What kept Wesnoth alive for twenty years is the same thing that kept BBS door games alive in the 80s: the content was community-created. Trade Wars 2002 was different on every board because the sysop configured it differently and the players made it their own. Wesnoth's WML scripting language lets anyone build campaigns, factions, entire rulesets. Most open-source games die because the core team burns out and nobody picks it up. Wesnoth survived because thousands of people have a stake in their own content inside it. The game is a platform, not a product.
I have built both monoliths and microservices over thirty-plus years, and the honest answer is that LLMs make it easier to generate more code, not better architecture. The hard part of microservices was never writing the services. It was debugging the network calls between them at 3am when one service silently started returning empty responses. LLMs do not help with that. They just help you create more services to debug.