If the extra enjoyment of a new RPG isn’t worth the cost to learn more systems until you find a better one, it’s rational to stick to D&D. Sunk cost fallacy is when you stick to it even though it’s not rational.
It’s probably worth learning some simple systems, but if you want crunch, is it really worth going through the effort of learning GURPS even though you don’t know if you’ll enjoy it any more?
Though the biggest problem is finding someone else to play it. Everyone plays D&D, so even if it’s not as good they’ll stick to that. I could learn a new system and enjoy it, but it’s all for naught unless I can find other people to play it with me.