I'm not sure why that would be the case. There would be only so many ships the PLAN could reasonably deploy compared to the number of ships that are sailing towards or within Asia at any given time - if it did establish a blockade ships going to Taiwan would not disclose their destination beforehand. They might only get one chance at an intercept, and having Taiwanese submarines running around would complicate things.
Also, because the submarines will get the same weapon fit as US submarines, the USN could also start sinking Chinese ships and plausibly deny it was them.
Besides, if Taiwan doesn't build the submarines it will definitely have one less tool in its kit to deal with a Chinese blockade.
How exactly would the PLAN run away from submarines it may not know the location of that are armed with UGM-84s?
The fundamental issue with going for SSKs, is that the ROCN is basically trying to take on an asymmetric strategy to target a "traditional" weak domain of the PLAN -- ASW.
The problem with this approach is that the PLAN have also been aware of this relative "weakness" for decades going back and have made massive advances in this domain, just as they made massive advances in AAW as well when AAW back in yester-decade was considered a "traditional" weak domain of the PLAN.
The idea of ASW being a "weakness" for the PLAN is actually a bit of a canard -- it's more accurate to say that ASW is just one of the domains of the PLAN that hadn't yet been developed to globally competitive levels, while other domains were prioritized first. After all, going back two or three decades, the PLAN was weak in every domain of naval warfare.
As part of their modernization process, first the PLAN sought to advance ASuW to globally competitive levels of the time, then with their AAW destroyers and frigates it was AAW, and then ASW.
More importantly, one needs to look at just what kind of capabilities the PLAN are building and which other opfors the PLAN (or indeed the PLA) are developing overall. For the PLAN, even if we leave aside whether the ROCN has a fleet of 8 SSKs or not, they were always going to eventually have a demand for a capable and robust ASW capability both in littorals and in open ocean because their major potential adversaries (again, this is not including the ROC military) had capable submarine fleets capable of operating both in the littorals and the open ocean as well.
And looking at the PLAN's procurement of ASW capabilities over the last decade -- not only in terms of ASW MPAs, but just look at the sheer number of surface combatants at all sizes (1500 tons, 4000 tons, 7000 tons, 13,000 tons) that are equipped with twin tail towed sonar suites and standoff range ASW weapons, as well as the development and inevitable large scale procurement of ASW helicopters (Z-20F specifically, but also iterative variants of Z-9 and Z-18 for the ASW role) -- and how much further ahead the PLAN already are with their own SSK fleet that is virtually guaranteed to continue expanding in quantity and advancing in quality.... going for a fleet of SSKs might have been a very logical and in-time procurement decisions in the 2000s, but for the 2020s the ROCN will be facing a PLAN with an ASW capability that in 2020 is already very robust and likely to be a magnitude more capable by 2025 when the first IDS is received.
The fact that it was always on the cards for the PLAN to have a requirement to develop massively capable ASW capabilities to counter much more capable SSK and SSN fleets from other adversaries that operate in the region, means the PLAN doesn't really have to do anything different in its foreseeable future procurement or development strategy to "counter" the ROCN's SSK procurement, because their sights from the outset would've been to target much more capable foes to fry much bigger and more capable fish anyway, operating in the same region in which ROCN SSKs will be operating as well.
All this isn't to say that countering SSKs is "easy" --- but I am saying that the ROCN's procurement will basically be going up against a PLAN where ASW will be far from the relative weakness they had in yester-decade and is likely to be as mature in 5 years as their AAW capabilities became by circa the mid 2010s.
For ROC big ticket procurement the issue is always about "opportunity cost". Now, procuring a fleet of modern SSKs obviously has some certain industrial, and political/PR factors beyond their military utility alone (which of course do definitely exist) -- but would the money and effort spent on the SSKs have been better spent on say, a larger fleet of individually smaller SSKs? Or perhaps a fleet of small draft, AShM FACs (smaller than the Tuo Jiangs), or incentives to train and retain personnel in the army?
These questions are rhetorical, they are just meant to point out that opportunity cost does exist, and for the ROCN with a limited budget and a PLA that's developing to fight foes who are much more capable than the ROC in the air and naval domain, that is an important consideration.
IMHO, ROC procurement if they truly want a more asymmetric strategy, should be to procure systems and adopt doctrines that avoid the PLA's current or future/inevitable strong points that are expected to emerge.
The PLA in the westpac is seeking to fight a high intensity air-naval-missile conflict and is acquiring all of the weapons, sensors, battle management, and EW systems for such a conflict, from distances between China's immediate littorals out all the way into the open pacific. All of the domains -- A2A, strike/standoff strike, maritime strike, maritime surveillance, AEW&C, EW, ELINT/SIGINT, air defense, missile defense, amphibious assault (to an extent), naval ASuW, naval AAW, and yes, ASW as well -- that will be incorporated in such a conflict are areas that the PLA will pump the most effort and funding and force development into.
If I were the ROC, I would try to fund much of my efforts into areas where the PLA is not actively focused on -- i.e.: basically it just leaves urban/irregular ground warfare. Yes, the ROC procuring Harpoon missiles and some SSKs and mines are a fair asymmetric strategy, but those are all domains the PLA would've been heavily working towards to achieve capability in anyway.
OTOH, the PLA is not actively spending massive amounts of funding into urban/irregular warfare on the ground combat and the requisite ground and air-ground support systems for such battles efficient, so that would make a prime area for the ROC to try and exploit.
... The problem is such a strategy basically concedes or accepts an eventuality that the PLA will establish a beachhead and land boots on the ground, and unless the ROC military leadership and political leadership are willing for the populace to be willing to actively fight and suffer for the cause, and be seriously willing for Taiwan's major cities to experience the likes of Grozny or Fallujah, such a strategy would obviously never be accepted.
In that context, sure, acquiring SSKs makes sense, and is probably a less bad decision than procuring M1A2Ts or F-16Vs, but it still suffers from the same problem of being "not-really-asymmetrical" vs what the PLA is actively wanting to develop/developed.
If I were the ROCN, I'd just hope that the PLAN's ASW capabilities to 2025+ doesn't undergo the same massive advancement in capability that the PLAAF enjoyed in AEW&+BVR in 2000s to early 2010s or the PLAN's advancement of AAW from the mid 2000s to mid/late 2010s.
