I can be a semi-casual gamer. The idea of much longer championships usually makes me feel like "man, I just want to clear this championship and move on to something else. The championship which seems like forever to eventually get done is the "1,000 Miles!" event where all four races are their own endurances. I sure as hell don't want to do 10 or 15 1,000 mile endurances in one championship. Ten races is usually a favorable number of races for most series (give or take). An interesting approach to more races could be if PD comes up with Special Stages in which points aren't on the line, but big pay days (or nights) are up for grabs. Maybe also think about an optional all-star race. The true benefactor would be more professional racing series. You wouldn't want to do a 15-race series with a series you can't win even if you maxed out your car's performance now, would you?
There's another possibility with extended championships. Think of most touring car racing series like the British Touring Car Championship or the Australian V8 Supercars in which you have a series of races spanning a day or two or three. You can imagine a championship for muscle cars around Laguna Seca or Sears Point for two days, right? If PD chooses to go with this format, then some championships may have one or two very long races apart from the sprint events. Think of the Australian V8 Supercar Series competiting in the Bathurst 1000. Think of NASCAR's Coca-Cola 600. So something like this could be possible.
I probably wouldn't want this for all championships. Ten races would be my maximum except the GT4 Formula GT series. More tracks and venues would be needed to host many more rounds of competition from places around the world. And look at it from a professional standpoint. Most series don't have extreme number of races because of maintenance costs and physical fitness needed for a full load. So that's why the ALMS doesn't do 30 rounds. I think for most gamers, 10 races is a very fair amount of races for gamers of all or most levels. Most casual gamers likely want to play the rest of the game and the rest of the championships. They don't want to feel like you're in a series taking seemingly forever to eventually complete. It's just me. Sorry if I seem like a true anti-Christ to any sort of professional stuff in GT5.