Its a little different now that you're just not replacing static parts. Sure you can replace a complex fuel controller and such in a modern car which means plugging a box into some wires, but what about something that is net enabled, requires some subscription (or active warranty) and is constantly being updated and monitored by Tesla HQ? Where do you get a new central computer for an old Tesla? How to you get it back onto HQ? Will you be allowed to?

Its a bit like building a PC vs having a smartphone, which is mostly run by the OS supplier and carrier. The former is doable but the latter is way too locked down for the DIY crowd. The phone-ifcation of all things is concerning from many perspectives.