People complain about new cars being harder to fix because of electronics but I think post OBDII they are actually easier because they tell you what is broken with error codes and also forums of experts are just a click away.
I do think its true that you can no longer be a diy hotrodder for most models because there is so much complexity with timing, emissions, fuel efficiency strategies, variable valve timing etc.
I've found most car forums tend to drive the real experts away due to them being full of blowhards. The real experts get tired of getting told by these folks they don't know what they're talking about, and end up creating closed invite only forums to trade info and advice on that the casual shade tree mechanic has no access to. I completely gave up on Rennlist after I asked a question about the turbo control system on the Porsche 951 only to realize many useless replies later that these people didn't even understand the basic theory behind how a turbo works. I answered my own question by bench testing an old junk turbo I bought on eBay and reported back to the forum only to be yelled at for not trusting their 'expert' advice. I've had similar experiences on other forums. Don't even get me started on the internet myths of DOT5 brake fluid that pretty much every forum takes as gospel. Even stacks of research papers and mandatory legal requirements don't keep people from spreading false information. Who needs all that nonsense? I now typically just pick up the phone and talk to experts I have personally worked with.
Funny you mention a 951 that is the exact car I am working on right now (an 86). Certainly Rennlist has been not very helpful, but I think its just in the nature of the internet to have an extremely bad signal to noise ratio. I use the term experts lightly but I find that the knowlede online goes way beyond what you can find in the service manual. Roughly 75% of the time I can find an answer and the rest I need to do some basic science or logic like bench testing a turbo.
When a car makes regular maintenance really easy and/or cheap, it's going to be more likely to never break. My Toyota Prado had the oil filter easily accessible at the top of the engine bay instead of being a PITA deep underneath near the sump, and had a "lip" around the seal to catch oil drips and divert down a hole to a catch line.
Makes me actively angry that this design isn't universal.
I do think its true that you can no longer be a diy hotrodder for most models because there is so much complexity with timing, emissions, fuel efficiency strategies, variable valve timing etc.