PC VR allows for more graphically-demanding games than an untethered standalone VR headset, assuming you have a relatively decent graphics card at least. However, Oculus added beta support for Oculus Link to the Quest last fall, effectively allowing it to be used as a PC headset as well when connected via a USB cable. So while yes, the Quest's built-in hardware is likely to feel dated before long, and will be limited to whatever games specifically support it, the device should continue to support newer, more demanding games when tethered to a PC, again assuming your system's hardware can keep up.
Compared to the Rift S, there are some other differences in ergonomics and specifications that may affect the experience a bit when tethered to a PC, but if you have any interest in also using the device untethered with standalone content, the Quest will allow for that, while the Rift S will not. Seeing as they are about the same price, that is likely to make the Quest the better option for many.
As for these headsets feeling dated for PC VR a few years down the line, it's certainly possible. Headsets will most likely be offering things like higher resolutions and refresh rates within this price range a few years from now, and perhaps features like eye-tracking and new controller designs. The newer PC VR games are likely to still work on these headsets though. Of course, it's kind of difficult to say exactly what PC VR will be like years down the line.