Nope, they aren't. My home PC hails from 2008, runs the latest Windows 10 update, and a range of software from early 2000s to 2018. It runs modern games, too (I like playing indie games that aren't too demanding).
Secret sauce? Modern SSD and a GPU from a few years back, plus an HDD to serve as a repository of all my data.
But even without the benefit or running Win 10 and hardware upgrades: I also spun up my old 2006 laptop the other day to rip a CD (my other machines don't have a drive), and guess what, FF Quantum runs well on it. Bring on your web apps, that XP machine will run 'em.
My point is that the PC's really don't suffer from the level of planned obsolescence that device like iPad do.
The PCs, ultimately, have one large window to see the online world with: the browser, and as long as that runs, the PCs don't lose any utility: whatever task they have enough computational power for today, they will be able to handle in a decade. The mobile devices, with their shift to the app-for-everything ideology pioneered by Apple, have thousands that are easily broken, prompting you to upgrade.