I'm sure many others feel the same way. Payouts would make no sense for those who glitched, as well, and have all the cars they need.
I, myself, went over the 20M only twice so far, and that to get two particular 20M cars I wanted.
I only harvest credits when I need to, otherwise I have more cars than I have time for.
The credits usually go into endless paint-jobs (I might paint a car six times before I'm happy with it. The few times it comes dead on first try is electrifying - becomes a special car. OCD?
) and also into tuning - such as bolt on upgrades, as well as endless wheel changes till I'm satisfied.
This makes paint chips important so I go after those Seasonals, too, when able to.
Otherwise credits don't interest me - I know exactly where to fish and haul in the jumbo shrimp.
20M cars interest me - I went after and won the Jag even though I already have one - I like two. I can ruin one, keep the other bone stock, color, wheels and all as PD tuned it. Another way to 'save' credits is to stock cars for a quick sale, and an extra Jag brings in a lot of loot when you're strapped for cash.
I played GT4 entirely offline to test the game's 'playability' offline, so Seasonals are not the biggest draw for me. I have torn into the guts of the game and there is enough to keep me interested Offline.
But, as you say, a new challenge, a new paint chip, an interesting car/track combo - as well as, I might add - a chance to race new rabbits again at a fresh track - these are very tasty morsels and I look forward to that, too.
My Friend's Leaderboard, for some inexplicable reason is full of top runners.
From all over the world. And quite a few from this site, too.
I just log in to stare in wonder at their times. My times are usually half a day behind.
OCD can be healthy if you're following two laws: (1) You're not distressing yourself. And: (2) You're not distressing others.
Actually, what more can anyone ask for?