The biggest boon would be for mobile devs to help avoid the 30% store tax, but this is also the reason why mobile Chrome and mobile Safari (especially Safari) tend to lag behind in major ways.
I don't think WebGPU is the biggest thing that's going to let mobile game devs start shipping on the web. WebGL 2 can do graphics well enough today. Other things hold the web back.
Monetization is the big thing that people always talk about. The lucrative interstitial app install ads that mobile games have aren't on web, and in-app purchases have a lot more friction. Handling large binary assets is also a place where the web is lacking and WebGPU won't fix that. Finally, there's a lot of ecosystem and tools stuff that needs to happen too. The engines need to add web platform support which goes more than skin deep. Shipping a game as one giant binary is not good enough for web, you need to split your code into dynamically loadable modules and stream your assets. You need Basis Universal compressed textures. Etc.
Fix all of the above and then web games can shine, with or without WebGPU. Realistically, fix monetization and the rest will follow. Big caveat though: I think many see the monetization techniques currently used by mobile games as exploitative. It's not clear that you could do web game monetization competitively without the same level of exploitation.
Going for GL ES 3.0 subset, when devices do GL ES 3.2?
Going for a MVP 1.0 with minimum overlap across Metal, Vulkan and DX12 instead of using their full blown capabilities?
In both cases, having to use native GPU debugging tools anyway.
Rather give the 30%.