Literally yes, it will never be that powerful, and that's intentional. The old way was so powerful, able to do anything, because well, it basically just ran your addon in the core browser's execution context. Way too scary to give add-ons that much power.

However I genuinely do want stuff that isn't yet implemented. For example I want add-ons to get access to Firefox's copy of the Public Suffix List, which is what you usually wanted when your add-on actually just treats TLDs as special. I just this evening sent a PR to PassFF, a popular password manager to do something crude as a stop gap while we wait to access the PSL via Web Extensions.