See, if the only enjoyment you get from Crab is accumulating resources, I can totally see how being maxed is a disappointment to you.
For many players that actually enjoy Booming, the resource grind was a necessary, but annoying part of the game. Once you are maxed, you can finally do all the things that are possible, without having to worrying about resources anymore.
That is a great point. While you are upgrading the game trains you to focus on getting the resources you need. It is a primary goal. Once you max, that goal is essentially eliminated. I think most maxed players that continue playing do so by creating new goals for themselves. Leaderboard chasing, hopping, mastering new troop combos, focusing on TF play, etc.
I still like to collect resources even though I am maxed. It gives people a reason to raid you, which can lead to more intel for your TF.