Or there are a lot of "only-for-that-particular-visor" games around ?
In practice, for the PCVR space you're looking at SteamVR games and Oculus runtime games. Every relevant PCVR headset can be used with SteamVR one way or the other. HTC Vive/Vive Pro (but not Cosmos), Valve Index are supported directly by Valve, WMR (Windows Mixed Reality) through SteamVR drivers provided by Microsoft. HTC provides official SteamVR support through their software a well.
The Oculus headsets are the odd ones out. They're supported in SteamVR as a pass-through, but by Valve and not by Oculus. But as long as you get a Rift headset up running in the Oculus runtime, it'll also work in SteamVR. Rift DK1 and DK2 still work too as far as I know (may need older Oculus runtimes, not sure). In practice, they just work and this is just an implementation detail, though.
WMR exclusive titles exist in theory, but I can't think of one right now. The official Minecraft VR client supports that and Rift I think, but Vivecraft is the better way to play
For the long run there's already
OpenXR that has been specified to replace the VR runtime fragmentation (well, fragmentation of targeting a certain runtime via one API to be precise). It's backed by every relevant party in the VR space, but so far there are only beta implementations for Oculus and WMR.
You may have noticed how the SteamVR headsets cannot run stuff using the Oculus runtime, which is partially true.
Revive exists as a wrapper to allow SteamVR-compatible headsets to run Rift software. It isn't always ideal, but works fine for most titles.
If the studio doesn't happen to be funded by Oculus, they almost always try to get as many users as they can and either support Oculus runtime + SteamVR or SteamVR.
So overall I'd say the hardware won't become completely obsolete or unusable for years to come. VR is rapidly evolving however and some titles may require some fancy new hardware feature of the future without providing fallbacks. Not terribly different from GPUs with 3D API extensions, though.
Just don't expect it to be an investment. Your hardware will lose value fast once something better comes along. VR is an expensive hobby, but I think we're past "VR has no games". There could certainly be more and longer ones, though.
Oh and I totally forgot to talk about Alyx in the other post. Currently playing it. It's really good so far.