The guy who wrote the tweet in the OP regularly posts publicly about the internal structure and politics at Valve. Reading through that gives me the impression that money is never a factor at Valve, and it's all about 1) whether anyone cares enough to work on a project and/or 2) whether someone is trying to impress/kiss-ass to climb the social pyramid.
I can't imagine the idea of rewriting CS:GO from scratch just to improve maintainability is going to get very far.
Plus players will 100% notice even the tiniest changes, and will complain about it forever. A game like CS:GO will never die, just look at the player numbers for its predecessor which has 6000-7000 daily active players.
