Technically you "can" run 4k exports in final cut pro on a 2012 13" MBP, but not only can you not run the latest version without a patcher, but it is so slow that you could take a nap while it's exporting, and it would only preview in a couple FPS. Technically you "can" continue running your iPhone 6S on its original version of iOS, but it would not support the latest apps, many apps just wouldn't run outright, the degraded battery would make battery life more terrible than it already was, and inevitably breaking hardware would be much more likely at that old age. Sure - technically you "can" do things on old devices, but I would call it a bold claim that it is worthwhile to save the money at that point.
You could do more things if Apple decides to continue to support old OS or install new OS on the old machine.
Without Ventura, you can't use family album in Photos, so you can't sync it with your iPhone.
Honestly, what's missing to a 2015 5K retina iMac with Skylake to support this kind of feature ?
Not everyone is exporting 4K videos ...
A lot of people use their Mac for office purpose, web browsing, photo viewing, for watching videos ...
For basic usage, a 2015 hardware is perfectly capable of doing any task.
The way the Apple APIs work force us to have the last system, they rarely back port their API changes to older OS. So when apps want to take advantages of new features, they need the new API and the new OS.
I have an iPhone 13, I do nothing more than with my previous iPhone 7. It's faster, but the 7 wasn't slow, photos are a bit nicer, but I can do better photos by learning photo techniques. The photograph is more important than the device.
Small thought:
What people do with Office 365 / Windows 11 (on a 4Ghz CPU, 8Gb RAM, 1Tb of HDD) they can't do on Office 97/Win 98 (100MHz CPU, 8Mb RAM, 1Gb HDD) ? For most people, they write exactly the same documents with the same features. But the required hardware is 1000x more powerful ...