Despite liking these types of game tech topics, i usually consider it very hard to talk about it online since the discussion always end up hijacked by hardware warriors. But since OP
Self
seems to be genuinely interested in the matter, i'll try to share my conclusions after doing some more research and reading some other opinions. Keep in mind that while i do have knowledge on the workings of hardware and low-level programming, i'm not some specialist or anything. The things i worked with weren't even gaming or graphics related. But lets go:
Are the new consoles going to be more powerful than the top gaming PCs out there?
Yes, on top of using the most powerful hardware available, they include a lot of technical advances to solve some previous existent issues bottlenecking hardware, most notably with the PS5's architecture.
Wow! So are PCs going to become obsolete and the ps5 will reign supreme!?
No. Even if the new consoles are advanced, its not to the point where absolutely everything else in existence with equal or slightly worse performance becomes automatically obsolete or unusable. The main aspects of next gen are: "Fast streaming of game assets" and "Ray tracing". Current PC hardware can already do both to a pretty good degree.
Ok, but are PCs going to hold back the new consoles?
Also no. For starters, AAA companies always develop with consoles in mind since its where the largest player pool is. Anyone who has been on PC for long enough knows big publishers always treated PC players as second class citizens. They wouldn't care about slapping "Needs a SSD" on the minimum requirement list and calling it a day. And quite honestly, based on past experience, PC players would simply buy the new component in the end as it always been the case.
If anything
current gen will be the thing holding back both new consoles and PC for at least the next 1 or 2 years, mainly because its where the largest population of gamers will be.
Fine, i understand. But its true consoles are finally owning all those annoying master race guys right!? Right!?
Yes but, also no. While its true
at the moment these new consoles can be considered the most powerful gaming device, and probably will still be at the end of the year when they supposedly release, some of these technical achievments
are going to make way to the PC platform, probably pretty soon. AMD, who worked with Sony in the PS5's architecture, for example, is already rumored to release new sets of hardware in the next year (or maybe even at the end of this year). It would be silly to think they won't apply what they learned working on the PS5.
There's also the matter of constant technological advance, as well as the way its possible to compensate the bottlenecks of a non-specialized machine with sheer power. Its also important to remember that, despite being general-purpose, a lot of PC hardware is still designed targeting the gamer public.
It wouldn't be weird to have a PC more powerful than consoles by the next year, and the general PC gamer population already owning equivalent or better hardware by the next 3 years or so.
Now, to answer OP's question:
How long will mechanical disks on PC be a bottleneck now consoles have SSD as standard?
If it dependend only on PCs, it'd be probably a day one change. What will bottleneck the new consoles for a while are the current-gen ones with their slow HDDs. After that, that bottleneck will probably only exist on certain types of games made for the masses, AKA F2P games.