It’s not the expense as much as not being able to get everything to work properly. For example, losing climate control readouts just to upgrade the HU just doesn’t sound that appealing. Still very much possible with some mainstream vehicles, but I can’t even begin to think of the bugs I’d have to chase down, if I attempted to upgrade the ML system in my Lexus. There just isn’t enough after market adaptors that would make things work. As far as my old Z4 Carver system, I probably could have upgraded it easy enough, but that thing would rattle your teeth wit two 6” subs. To get the same impact, I would have had to take up every cubic inch of the shepherd box trunk with amps and a box. I would have lost all storage, added weight, killed handling and gained....well, not much. The decks in those cars would go out pretty regularly, so you will see tons of after market decks. So long as the car didn’t have a Carver system, they weren’t a complicated upgrade. By that same token, if they didn’t have the Carver system, they weren’t worth keeping. In 09 BMW changed the audio system with the release of the new model and things got complicated. It’s just how things have gone.
If there’s a will, there’s a way, but as mentioned here, I could by 3 to 5k worth of home audio equipment, hit the power switch and bask in the glory of my excellent components, or spend double that and spend the next decade trying to figure out why things don’t work as they should. Up to 6 months ago, I still had old Orion amps from the 90s, an Audio source crossover, an Orion Xtreme 12” sub, Polk component drivers and all the goodies I could need to fully outfit another car and never found the Adaptors I needed to put them in any of my cars since about 2003. They just got more complicated as they go newer. It wasn’t the expense for me, it was the technology matchup challenges.
If anyone wants to do it and have a mainstream car, where conversion accessories are available, I don’t see why you wouldn’t if that was your thing.