You said you have 1500 usd budget.

Even if the cpu you can buy now will survive next gen , are you ready to pay 500 usd more to upgrade the gpu for next gen ?

You must also understand that you don't really need a modern pc to play this generation games. I play pc games at 1440p High Settings on a 2020 PC.
Since you have Playstation 5 & Xbox i don't see any reason you cannot wait to avoid the extra 500 usd gpu upgrade for next gen.

(except if you play a lot of games and you want to go open seas route, to save a lot of money.)

$500 for next gen sound like a solid deal to me. That’d be less expensive than a PS6, I’d imagine. Maybe I should clarify that it’s only $1,500 for the initial purchase…

And the reason I’m looking to upgrade to PC is mostly for older games. I feel very restricted in the PS/NSW ecosystem when it comes to playing any game I’d want: For instance, where’s Sonic Unleashed 4k 60FPS on my Switch 2 or PS5? I’m gonna open my unused Xbox for that one game. Similarly, where can I play the highest quality, up-res’d, 60-120FPS version of Mario 64? And that’s not even mentioning all the fan games and mods I’d have access to.

All in all: I’m better off just letting go of my >$1,000 worth of PS5/XBSX hardware alongside their online annual subscription and jumping over to PC ASAP.

You're at 1,500. Hang on, save another 500 and build a great rig instead of a good one. Idk much about PC's these days but I know 2000 will get you a massive jump over 1,500.

It depends on the CPU and RAM type/speed that will end up in PS6. It's always easier to replace a GPU, yet if PS6 is using new RAM or a 16 core CPU then you might need a new CPU / motherboard as well. 

But you're likely fine with an 8-core Zen 6 CPU and DDR5 ram, DDR6 ram is not available yet

DDR6 is a future technology expected to be released in 2027. DDR6 will require new motherboards and chipsets specifically designed for its architecture, just like past generations of RAM. You will need a new motherboard to use DDR6 when it becomes available, as it will not work in existing motherboards that are designed for DDR5 or older types of memory.

PS6 will likely have GDDR7 RAM which is GPU type RAM (much faster than DDR5 and DDR6) which the PS5 can both use for system and graphics. That's where PCs are at a disadvantage with split RAM. (DDR5/DDR6 have lower latency for system tasks, GDDR5 and GDDR6 have higher bandwidth for GPU tasks)

DDR6 is likely better to keep up with PS6 than DDR5 (DDR6 will use a quad 24-bit sub-channel configuration (four 24-bit channels) instead of DDR5's dual 32-bit configuration. This increases parallel processing and bandwidth efficiency) but it is as you said, if you keep waiting for the next technology you never get around to buying a PC...

And while you might not 'beat' the PS6 in every way with current tech, you will easily be able to play all its games with just a GPU upgrade down the line. Just make sure you have enough room for memory expansion with the rumors of PS6 getting 24GB, 30GB or even 40GB of RAM.

Yet the games will still be made playable on 'lower' end systems, you only need those kinda speeds and memory for 120fps 4K/8K native etc.

"to get PS6 games performing decently smooth"
Yes definitely you can buy a PC now and play PS6 games at 1440p60 at least.

A 2020 PC is not modern? :p

I bet older PCs can still keep up. 

AI cuts it off at 2017:

I understand you , i have an imba collection of pc games of around 20 terabytes plus emulators. :P

I don't know what cpu playstation 6 will have m8.

If we speculate it will have a 8-core, you can gamble and buy one of the best 8-cores cpu on the market  + a medium gpu around 300 usd (2060 super performance is enough for 1440p).

For monitor if you have a big desk and can place it 50 cm distance away go for 32 inch is the perfect size for gaming. (If you have zero distance from the monitor then 27 inch).

Do whatever you like , but it is still a gamble because playstation 6 could have 12-cores cpu or something.

In 2 years if playstation 6 has 30 teraflops performance, my pc won't be modern for sure. :P

I could upgrade my gpu but i am afraid that my ryzen 3600 will eat the dust and i will have to replace the whole pc. ^^

Compared to PS6 yeah. If AI puts the cut off at 3 years prior PS5 launch, then 6 years prior PS6 launch hardware likely won't cut it.

PS6 will have 9 or 10 cores, 6 cores might struggle.

The PS6 is expected to feature an AMD "Orion" APU with a total of 9 to 10 Zen 6 CPU cores, consisting of 7-8 high-performance Zen 6c cores and 1-2 Zen 6 LP cores dedicated to the operating system. This architecture is designed to free up the high-performance cores for gaming and multitasking.

However if you're content with lower fps, you can still play ps6 games on that. But they'll look and play better on the console. 

Choices, new motherboard/CPU now or wait for DDR6 :/ And of course it's not going to be cheap when it releases. Trying to future proof a PC can easily turn into higher costs than upgrading a bit more often. Prices aren't linear between low-mid-high end PC hardware. 

I think i will have problem for sure, i choosed 6-core instead 8-core so the gpu will not bottleneck due performance.

Imagine what will happen with future gpus with 300% performance of mine. :P