Why does the game need to keep advancing? Most games in the industry are written, released, and finished. The best have a timeless nature, still excellent even though they haven't been changed for decades. Battle for Wesnoth is an excellent game. But declining interest seems to indicate that maybe it is just finished?
I regularly play Wesnoth over the internet with a friend. Though we sometimes explore other games, we always come back to Wesnoth because it's compelling. However, and I say this as a fan, this game has severe issues - it's not just a few bugs and maintenance: Wesnoth makes current hardware sweat to the point where my relatively new MBP sounds like it's taking off, as is my friend's ThinkPad. It's also the only game…
Getting it running on an existing engine (UE4 can do 2d fairly well, for instance) seems like a better investment of resources. What they're building is a solved problem. It was a valiant effort to do it on their own, especially in a time when that was the only way. They could reap the benefits of all those learnings in the backend and instead focus efforts on tweaking gameplay, inventing new forms of play, and smoothing out rough edges in the UI or visuals.