After almost 9 years, of using my self built PC. I bought a prebuilt PC. Should I keep it?

I don't understand why the PSU would not be updateable

I'd hang onto it at least for a couple years once DDR5 start becoming more available for modern builds, and CPUs and MOBOs manufacturers start adding more support for it.

Sadly, since this whole pandemic started, everything has been messed up and skyrocketing prices along with it. Perhaps when more people get vaccinated and feel more confident in trying to get back to "normalcy" (whatever that is 😛), maybe prices on computer hardware will start dropping by then! 🤞

EDIT: Damn, I don't know if this comment is relevant anymore since you added a lot more stuff to your OP!

Wouldnt you be able to upgrade the PSU by using a new Case?

Depends a lot on the type of games you play, and at what settings. 1060ti should be good enough for a lot of games for quite a few years. However, the latest GPU intensive AAA titles might struggle on it depending on the setting.
However, due to inflated GPU (and PC parts) prices rn, I would not recommend building one. Sure, you might also get a better price for all your older stuff if you sell them today, but at least in my local market you would still be paying a premium for the new stuff. So, unless you can not hold out for say about a year, keeping it would make more sense imo.

If you do 1080p gaming, you'll be perfectly fine with a 1660ti for awhile, so long as you don't always expect ultra settings.

My first thought was return it.
After checking the current prebuilt market: keep it.
If the bitcoin bubble still hasn't burst in the future, you could always flip the 1660ti.

I guess it depends on what you want to play with it and how long you want to keep it with no updates and of course which resolution and details you want to have. I wouldn't buy a Dell, but for your plans ("a few years") this should be working ;)

I would keep it. The price of $1000 for a 3060ti and 500 W PSU is not worth it in my opinion.
Even though the 1660ti might only be good enough for 2 years, by that time there will be a 4060ti / 5060ti and maybe there will be an upgrade possible that can handle the 360W PSU.

damn 1660 ti is something I'd wish I had :D Even if RTX cards dropped in price they would be out of my budget sadly.. But for todays prices prebuilt systems are most affordable now.

But as for prebuilt PC itself- don't these have manufacturer specific parts in it ? Meaning you can't really upgrade the parts or remove and sell them. 1660ti as well - I'm not sure of course, but in such a prebuilt case wouldn't the 1660ti be Dell specific card that works on dell specific MOBO ?

Anyone smarter about PCs please do correct me if I'm wrong, but I have read that parts in these systems aren't the same as those you can buy seperately, meaning you can't really expect to flip a part later on.

edit : I see that you already mentioned PSU and MOBO differ, but I'm pretty sure the RAM and GPU does too. Anyway - as far as performance go I would definitely keep such a PC while the world is complete mess and you can't buy anything. I suspect the Suez canal drama will only halt production and sales even more.

Can I ask you if you see a big improvement going from i5-3570 to the 10400F? Probably it's hard to tell since you also have a new GPU.

My current PC has i5 3570 as well and I'm thinking on upgrading to i5 10400F, but I'm not sure if I'd better get a new GPU instead (RX570 4gb is what I have atm).

Why not? I have this GPU and it is enough for 1080p.

A quick look at ebay shows the rtx 3060ti going for about $1300. You could even get the 3060ti pc, sell the gpu, buy the 1660ti pc back, and now you have two pcs and one gpu. Or sell the 3060ti and buy the prebuilt again (with another 3060ti). There are tons of ways you could go about it. This, of course, considering the $1300 price. Not that I like or support this gpu overpricing scheme that is going around, but such are the times.

Yes it is enough for 2-3 years, but it's not very future proof especially if you want to upgrade, and went for Dell with it's quirks, the prices for things atm are expensive, but in 2-3 years hopefully it will be better again, if you then buy something just save up and get something way way more futureproof and not something like Dell or Hp but something you can customize, better to splurge all in once and be done with it for a while then keep upgrading (and find out you can't upgrade certain things) and cheaper in the long run, that's my opinion atleast.
That's why i went for a coffee lake i8700k (didn't go for 9000 because of much more power consumption and little bonus to gaming, and a 2080TI) however if DDR5 comes i would still need a new motherboard.

Also if i go ultrawide screen i don't know if my cpu will bottleneck it, or a faster CPU would make much difference (with the GPU i already have).

Also consoles are nice and all but you rarely see the good game deals, the bundles you could get on pc and steam, also with the ps5 a lot of games increased game prices to 70-75 euro, you have to take that into account too, because it easily racks up money too.

i'd keep it.
right now in a lot of places prebuilts are the only way to get a new computer for a reasonable price or even get one at all.
chances are it will stay like this for the entire year or even longer because demand is insanely high and supply is pitiful thanks to the china virus.

a gtx 1660 ti should be enough for 1080p gaming in the near future.
and when it comes to the psu you will have to take a closer look. it could be some special form factor. also large system builders like dell & hp are moving to the new 12vo standard (12v only) and i doubt you can find one of these psu in the wild easily.

I hate Dell and their proprietary stuff that can't be upgraded, that PSU is terribly small but in this climate, ugh, guess you should keep it.