Crowdfunding open source seems like a good idea in general (in fact I could see a platform built around that specifically), but you may be underestimating the amount of money/effort required to build something comparable to Photoshop. So I like the idea in principle, but not in this particular instance.
For what it's worth, I think Sketch is much more user friendly and is $99 one-time. Certainly not OSS, but a better deal than Photoshop for most things, and a much more user-friendly interface than Gimp (though I suppose that's subjective).
The logistics themselves are already giving me a headache. You have $400k, but are Gimp's developers (and they exist, right?) ready to just get an infusion of somewhere on the order of three years of salary and get to work? I assume the roadmap would change with that amount of funding.
There are crowdfunding sites that either just happen to be used for these things or are focused on them, the issues are complex. Here's a run-down (that needs a couple updates like including Open Collective): https://wiki.snowdrift.coop/market-research/other-crowdfundi... (on the wiki for a still in-progress platform designing a new funding approach around the specific challenges of free/libre/open projects)
Sketch is 99 dollars a year. After the year you can still use it but you won't get updates. If you work on a team you'll pretty much be forced to always have the latest version.
Still, 99/year is a drop in the bucket. Even 50/month for all of creative suite isn't all that much considering many of us will just add it to our business expenses and write it off.
For what it's worth, I think Sketch is much more user friendly and is $99 one-time. Certainly not OSS, but a better deal than Photoshop for most things, and a much more user-friendly interface than Gimp (though I suppose that's subjective).