There's literally no situation where it won't hamstring the PS6. None. And thicc, you know better than to suggest otherwise.
That shit with the Series S? The extended cross gen period? Pass it along and triple it. Thats the degree of negative impact we're talking about here. And it's not going to be a net gain for the ecosystem: Again, we heard that with the Series S, but we also heard it about PSVR2, PC ports and the PS Portal. Well, the PS5 is behind the PS4 in sales and I think it's going to finish behind the PS4 even with GTA6 and despite an Xbox that will be cut by almost half compared to last gen. That's a userbase contraction.
It doesn't even make logical sense. All 10 PeeSea pirates who bought a Steam Deck or ROG Ally are not going to abandon their libraries for a PS handheld. PlayStation does not (or at least shouldn't) make the kinds of games that anyone wants to play on a handheld that compromises the experience. Most people who use the Switch in handheld mode do it within the confines of their own house. Same goes with Steam Deck. If Sony is so desperate for a handheld option, they can stick to making Portals.
OK, hear me out....
Bringing up the Series S, yes that has clearly been a bottleneck for Xbox this generation, and it's held the Series X back. But we're also talking about 2020 tech with no form of actual hardware-dedicated ML/AI upscaling, and a bloated SDK with so many API choices it actually led to less optimized games vs. more. Those aren't going to be problems for a PS portable. Sony & SIE aren't going to hamstrung their 10th gen systems by just ditching PSSR or cheaping out memory configurations just to save a few bucks at the expense of long-term performance through the gen. They won't make the PS6 and the rumored handheld on different technologies, and that handheld isn't going to have its own dedicated software library so it won't risk siphoning resources from PS6.
A PS portable is a way different thing vs PSVR2, because the market for large, socially isolating headsets in general isn't very large, let alone for gaming. VR won't have truly mainstream appeal until they get that thin, fashionable iPhone-like moment paired with real software ecosystem support, so that's a ways off. PS Portal hasn't really been a detriment to PS5 I feel, because it's just a streaming handheld. Devs don't have to do anything on their end to get games running on that thing.
The PC ports, I agree those are likely causing some gradual build-up of long-term damage, particularly because of all the non-GAAS which have been ported already plus the fact their GAAS strategy was partly in favor of having more games on PC, and we've seen now a lot of that has come at the expense of non-GAAS 1P one way or another. But we won't really know the impact of the PC strategy until PS6 launches, and that also depends on if SIE haven't already internally shifted to a more sensible (scaled back) strategy on that platform going forward.
Talking about userbase contraction though, the truth is unless there is a PS handheld next gen, there's going to be a contraction regardless even if SIE does everything right and avoids the mistakes they made this gen. That'll happen due to pricing trends likely being reflected again 10th gen (i.e no real price cuts, potential price increases throughout the gen), continued growth of certain PC gaming platforms like Steam, and less exclusives (1P due to higher costs & longer dev times for AAA, potentially fewer AA than should be, so less releases; 3P due to less reason from 3P and platforms like Steam & Switch 2 getting more preferred Day 1 treatment). Also it's clear in places like Japan non-portable consoles are shrinking in appeal, and Japan is still a larger market for core console gaming than many singular European, LATAM, MENA, African and even many other Asian markets like South Korea and arguably (at least for now) China. It's not the write-off some people want to pretend it is.
So knowing all this, how is SIE hurt by having a dedicated PS handheld for 10th gen? They can develop tech that can mostly automate resolution downscaling (basically take what they're doing with PSSR but apply algorithms that do the inverse of upscaling) and auto-simplify LODs to some degree. Maybe do partial framebuffer updates combined with blit operations on selective pixel data (I think that'd require having hardware layers for the framebuffer, think like how systems like the Saturn had multiple layer planes....but that might require devs to re-learn approaches they probably haven't used in decades if ever). The important part would be, making sure the PS6 and handheld have the same features on the CPU & GPU, and having tech & APIs in place that can automate as much of the process as possible, so hopefully devs don't need to do more than just add some metadata scripts to their code to exploit the hardware features.
In short I wouldn't use the Series S as a cautionary tale or foreboding against the concept of a lower-power companion system in a device ecosystem, because the Series S wasn't a well thought-out system at the engineering level in the first place. And, I mean that in terms of it complementing the Series X without acting as a bottleneck, or having a market use-case that didn't directly conflict with or become redundant with Series X's. Series S failed on both notes, but that doesn't mean PlayStation would.
EDIT: Could also put it the following way. Just because "mid-gen" upgrades like the 32X failed, doesn't mean the concept of a mid-gen upgrade itself was bad. It just meant SEGA's implementation of that idea was not well-realized.
It's the same thing here with Series S & X, and a multi-SKU concept with systems of differing performance abilities coexisting within a generation.