You misunderstand. Before tetraethyl lead was removed from gas--in the 70s, I think?--engines were not nearly as good as they are now. My dad was doing really, really well to get 100,000 miles out of a car in the 60s and 70s; you used to see a service station attached to every single gas station, because of how much service cars needed. Now, 200,000 miles is close to the minimum that people would expect with only preventative maintenance. It's nearly unheard of for people to need to replace valves and regrind valve seat now, except for high compression, high RPM engines (esp. supersport motorcycles). But that was just normal before the mid-70s. My dad has done multiple full teardowns on engines before the 80s, replacing head gaskets, piston rings, valves, and so on. These days that's almost unheard of.

I think that the most intensive valve maintenance that I'm aware of that's common right now is cleaning carbon off for some of the direct injection engines. I know that it's an issue with Volkswagon cars, but most cars don't do DI. You'd have to check technical service bulletins (TSBs), but most cars are very trouble free compared to what you could expect prior to the 80s.