I don't agree with the financial aspect.
Well, you are spending for more than what you need, on the off chance it will make it more futureproof, but since what you have now meets your needs, by definition you are overspending. If a 2010 machine was even remotely viable, then a $499 Mac mini would easily meet and massively surpass your needs. And then you can keep the extra $ in your pocket and have it for other retirement needs, or you could spend it later on a “better” machine from Apple. Since ‘future value of money’ and having money accessible to you now is better than not having it now, particularly for retirement - well, hence my comment.
To put it another way, 150% more $ spent now gets a fairly incremental upgrade in the grand scheme of things if the base model already massively addresses your needs. In 5 years, the difference between the models will be seen as minor, but that $750 you kept in your pocket will easily buy a nice upgraded Mac (and you can sell the old one for $). Or can be used for other things in retirement. There’s a theme here….
Over a long period of time, the computer "pays itself". And I hate to upgrade anything, transfering all the files, update apps, reconfigure the preferences, etc. Once a good computer is able to do what I want it to do, then that's it, I keep it until it brakes.
Are you doing paid work with this computer? How does a computer “pay itself” ?
Perhaps you’ve not used Migration Assistant to migrate from old Mac to new, but the process is trivial. If that’s your fear, don’t worry about it; it’s nothing now. Turn on old Mac, run Migration Assistant, boot up new Mac, put both on network, let them talk for a few hours, done. It’s wonderful.
When the old 2009 mini had an HD failure, I tought about upgrading its components, but it wasn't worth it. I had to change the HD, upgrade the ram, reinstall everything, and I coulnd't change the old Core 2 Duo anyway. This computer was too slow at the time of its death, and the 2010 MBP was better (more ram, faster GPU, ...).
Those are trivial upgrades. 10 minutes and a screwdriver…
IIRC both were C2D / nVidia 320 based, so I’m surprised there was that much difference, but putting an SSD into anything breathes new life into it.
The security risk of an old UNIX system isn't really a problem, contrary to what many ppl here think. I was well aware of some risks, but I was pretty confident about this system, and like I said I never had any issue with SL during 12 years (this is a very long period of time!). But anyway, it is time for me to change my desktop computer since I need to do things that the old 2010 MBP is no longer able to do (web browsers are too old and can't be upgraded anymore) or because it is too slow with a few things (notably some Mathematica calculations). Also, my 17 years old Apple 20" display (from my previous G5 Mac Pro) is now showing its age. I need a better and larger screen.
You are wrong in what you say about security implications. Point blank, full stop. I suggest going to
https://www.cvedetails.com/vulnerability-list/vendor_id-49/product_id-156/Apple-Mac-Os-X.html here and reading for a few minutes. You’re essentially a decade unpatched, give or take a few years, so you’ll see lots of nifty exploits.
I don’t include the display in any calculations because you could resolve that issue completely outside of the question of whether to replace a 2010 Mini, and/or what to replace it with. It’s neutral to this discussion.