Most of what I want to build is likely to be short term. Maybe something will stick if I like it enough, but things I want to build have a lot of functional overlap and I don't have space for them all, so they can't all exist at the same time.
I'm pretty interested in seeing how different combos will perform with different software. So I'll start with one thing and then keep changing parts/drivers/settings gradually, logging benchmarks as I go. That process gets tedious (especially when I find I've made a mistake and have to start over) so it only happens in short bursts, but filling in the data is satisfying for whatever reason.
This can lead to buying extraneous hardware just because I want to experiment with it. Afterward it will add itself to my pile of junk, awaiting the day that I might want to play with it again, if ever.
I have one build in mind that I intend to be permanent, and which I would actually play games on - but who knows if I'll actually stick to it.
A 2nd system has been the basis for the above type of benchmarking and screwing around, but it *might* eventually settle to a permanent configuration. Maybe.
One thing I want to do is recreate an old PC I had in the late 90s, but then I want to modify it. When I look back it was one of my favorite systems, but when 3D took over it was suddenly too slow. That was a time when there were a lot of options for upgrading. So I'd like to experiment with alternate upgrade paths from what I actually did back then and see how satisfactory they would have been.
To make that more interesting I hope to find more historical pricing info. I've found 1 or 2 sources via archive.org but the info is too thin and scattered to really be sure what specific things cost at a specific time. The sites I used to shop from in the 90s are not in there.
But once I've satisfied that curiosity, the system will be redundant so I won't have any reason to keep it. The only reason (and best reason) for me to change my mind will be if I find I just happen to "like" it too much and prefer using it over a machine that supersedes it.