Last time I gave that a try (without LLM assistance though) it was really hard as games DirectX calls cannot simply be glued to WebGL so performance was bad.

Probably. There have been dozens of examples of taking old games (Crazy Taxi, Super Monkey Ball, Quake, etc) and making WASM browser equivalents using AI to decompile them just on "Show HN" alone.

They often ship the original assets in a somewhat brazen disregard for basic copyright law even when the games are still for sale on places like GOG though.

I saw recently that someone had ported Halo CE to the web and had 1024 players in Blood Gulch.

Definitely, TW can "cheat" a lot with using sprites/very low LOD at distance, only streaming the corresponding units on battle load, and it's turn-based so performance isn't critical. Although obviously you won't be doing those 50000 unit battles on the web.

Off-topic sidenote: what's with all these new projects targeting WASM instead of native, even if packaged for desktop anyway?

GTA V was ported to Wasm so everything is possible at this point

There's a Super Smash Bros. Melee website: https://lucasigel.com/melee

Probably would work although if you release/distribute/allow others to use it, you're treading into copyright infringement. Afaik personal use if you own a properly licensed copy is usually okay.