Actually I would say the same logic does not apply to 4th gen and 3rd gen fighters to the same degree.
Flight hour costs of 4th and 3rd generation fighters relative to trainers were not as exorbitant as flight hours costs of 5th generation fighters are relative to trainers.
Furthermore, when 4th and 3rd generation fighters emerged, the sophistication of simulators was significantly below what we have now.
"If you can afford them" -- IMO is the key here.
Sure, I agree that every air force with 5th generation jets would love to have twin seater combat capable trainers for 5th generation fighters.
But even if they were willing to develop them, the money to actually fly proper 5th generation fighters for the purpose of training, when a 5th generation fighter includes all of the bells and whistles like RAM, internal weapons bays, advanced full sensor suites, advanced networking capability -- is a waste of time of a 5th generation aircraft's flight hours.
There's a reason why 5th generation aircraft fleets around the world do not have dedicated 5th generation twin seaters for training purposes, and there is a reason why the idea of J-20S being used as a trainer is also immensely unlikely -- it's just a poor use of an air force's finite resources, and the training for 5th generation jets can be better done through a combination of modern simulators and LIFT.
Or, putting it another way -- I think that having 5th generation combat capable trainer aircraft would be an active detriment to an air force's 5th generation fleet.
I would truly question the rationale of the PLA if they developed J-20S for the purpose of training, and doubly so if they chose to do so for J-XY as well.
The structural provisions for a second seat are not there, however.
The canopy/hump that we see externally is an external aerodynamic shape that internally likely carries fuel, but even if they wanted to potentially make it a two seater in the future, they would have to make substantial structural changes below the airframe's surface to accommodate a cockpit and the requisite life support system and enlarged canopy etc.
This isn't even starting to think about just how big (long especially) a second cockpit would make the canopy, which, even on cursory glance at the aircraft, would actually require an even more outward displacing and longer hump than what we see now.
All of this is to say -- I'm not ruling out the idea that maybe we might one day see a twin seater J-XY (with the caveat that if we see a twin seater, it would almost certainly be for the battle management/control role rather than for training).
However, I am saying that the nature of the airframe modifications -- including the canopy/fuselage hump -- we see on this J-XY prototype relative to the FC-31 airframes, can be best attributed to the requirement for greater internal fuel and any possible aerodynamic benefits as well.
Whether those modifications might make developing a twin seater variant of J-XY slightly easier in the future if they chose to do so -- maybe, it does. But the primary and most upfront benefits for them would be greater internal fuel + aerodynamic refinement.