That's exactly what I was thinking before I bought my current 2020 Mac Mini.
I was trying to find a decent 2012 i7 Mac Mini that I could shove 16GB RAM and a 1TB SSD into; but the asking prices on eBay were stupid and no guarantee that they'd be as described when it landed and no one had one on the forums
TBH it made sense to go back to the latest i5 Mac Mini as it's new so will be supported for years to come; the 2012 will get supported until next year and that's it done, no further updates etc
As I use mine for personal and business use; it makes sense to have one with warranty and plenty of support!
The 2012 model, particularly the quad-core i7, have come to be recognised as arguably the best combination of performance and flexibility, certainly superior to the 2014 'downgrade,' so like vintage Sansui amps, have come to command premium prices.
You can still find the odd one and beef it up yourself, and for many tasks the i5 is perfectly fine. I have an i5 with an SSD and 8GB of RAM as a music server, should the i7 in the office burst into flames then the i5 would be a perfectly adequate substitute in the short term at least.
I may look to pick up another i7 myself in due course. There are a handful of Mac dealers who specialise in refurbs, so you might pay a bit of a premium but at least would potentially have a warranty behind you.
Dunno what you do, but my background's in design & advertising and more recently the web, and while back in the day (30 years now) it was necessary to be a 'power user' with regard to Mac specs, pretty much anything in the upper half of the range in recent years is perfectly adequate, at what, relative to 1990, is a fraction of the cost.
The only thing I ever ask the i7 to do which causes the occasional gasp for breath is rendering video, everything else it just breezes through.
Pretty much the only time I ever hear the fan is when Google's shitty Mac citizen of a web browser displays its long-standing memory hog/leak behaviour but that's about it.
As I said, I'd quite happily continue using this machine for the foreseeable future, a bit more RAM would be the only item on my wants list. Well, I suppose the onboard video's a bit pokey, but I'm emphatically not a gamer, and the days when I needed a Mac Pro with multiple video cards and monitors are long gone.
I'll see what they do with an ARM processor in the next Mac minis, I'd have thought those would be one of the first candidates to make the jump to Apple iron.