I've tried to understand what makes this so incredible impossible to maintain by asking people, I feels like FUD from the FF community (which I'm a part of) because it's all just wishywashy statements that it'll be impossible to maintain.

The reason all the statements are "wishywashy" is because it's impossible to say anything concrete here, we don't know what Google is planning to change in the future. But we know that Chromium is a huge, fast-moving code base, and that dynamic web request hooks require support from core parts of Chromium. If Google refactors or rewrites those core parts in a way which makes them incompatible with per-request dynamic hooks, keeping around v2 support means carrying huge patch sets against core parts of Chromium, forever. That's likely going to be a very large ongoing maintenance burden.

But Google haven't made those changes to how web requests are performed yet, so it's impossible to say how difficult it will be to add back whatever functionality is necessary to add back dynamic web request hooks. Maybe it'll turn out to be relatively easy, and maybe Google will leave that part of Chromium relatively unchanged for a long time. Only time will tell.