What can't you do? Literally all the game content is there 100% of the time. Every race you had fun in is replayable. Custom races mean you can make any race you want and drive any car against any others, or none at all. The AI sucks, so that's why you jump into sport mode to play daily races, participate in online TTs, join the manufacturers cup or any other special league of the moment. That said there are many races where the AI is a formidable opponent, and there is also Sophy there that provides a pretty awesome and aggressive challenge for those who want more.
The weekly challenges provide extra incentive for doing some of the campaigns races again, and most weeks there are special week only events too. If I had the time I could have spent hours upon hours in just last weeks TT with the Porsche at the Nurburgring, perfecting driving my style and scratching for tenths to stay with the fast guys over in the TT discussion.
If that doesn't float your boat get on here and look into racing leagues and servers that host specific events... there is always stuff to do if you love cars and a challenge, and tons of awesome GTP members putting real time and effort into making unique, cool, fun, competitive lobbies where everyone has a good time.
For me, most of my limited time is spent in the TTs competing against myself and all the people over in the Time Trial thread, creating custom cars and liveries, and doing the weeklies and the occasional daily race... and sometimes just chilling out driving around in VR checking out the details of whatever track I choose. I don't have a lot of time to allocate to GT7 in my life right now, so even getting some of those things done within a week can be a challenge.
Overall, GT7 is what you make of it, and in your case you specifically say you've got thousands of hours into the game. At what point is $60 not justifiable for that insane amount of entertainment? At what point does your play time allow you to try and manipulae the feelings of someone who simply said they were having a good time with GT7? We all agree that there are many flaws in GT, but it's still amazing, provides hundreds of hours of worthwhile play, and is worth every single penny you and I put into it.