*Sigh*The problem with what you're suggesting is that ZTZ-99s and ZTQ-15s aren't likely going to be operating in the same terrain. The ZTQ-15 is a mountain tank designed for high-altitude operations, while the ZTZ-99 is a heavy MBT that's intended to take out Abrams. When you take the ZTZ-99, shouldn't it end up with the same problems as T-90s as engine power gets suffocated by the high altitude?
I can also accuse you of obsessing excessively over equipment designed to function as a system of systems.
The problem with system of systems is that they are easy to disrupt, for example, sufficient jamming power can ruin the synergy between various parts of the system of systems and take it down; consider how the Americans are aiming to counter the DF-21, for instance. They're not thinking simply about hard-killing the DF-21 with an interceptor missile, they're thinking about disrupting the entire kill chain, such as shooting down satellites so there's nothing available to guide it, or jamming it, using smoke, decoys, etc.
Ideally, you want your force to function as a system of systems. But in actual combat, against a non-trivial opponent, they will make attempts to disable your system of systems by isolating and eliminating vital support elements. For instance, SAMs can be used or Indian anti-tank missiles can be used to knock out WZ-10s, as most gun-launched ATGMs have either a primary or secondary role of countering helicopters. ATGM IFVs, likewise, are highly vulnerable to artillery as they're rarely well-armored. When that happens, the side arms should come out and your soldiers and equipment should still have options to continue fighting while attempting to withdraw.
It's like saying, infantry have difficulty wielding heavy ATGMs like the ones on gunships, so they should just rely on vehicle support to counter tanks. Instead, infantry usually have anti-tank teams utilizing top-attack ATGMs of some kind, even if their capabilities are reduced compared to heavy ATGMs emplaced on vehicles. It's not going to make them a hard counter to heavy MBTs, but it gives them some level of defensive capability if they can't get vehicle anti-tank support.
If you did follow some PLA news you would have found that they indeed brought ZTZ99 to Tibet Plateau in June.
EVERY modern Chinese tank model must pass trails on Tibet Plateau. ZTQ15 having better plateau performance doesn't mean that ZTZ99 should have severe engine problems.
And also, if you cannot expect ZTZ99 to operate in the same terrain as ZTQ15, then why insist on the “ZTQ15 vs T90S” scenario? Given that the power/weight ratio of T90S is similar to (or lower than) ZTZ99/ZTZ99A, if ZTZ99 won't operate then neither will T90S.