While it would almost certainly cost more than 400k to build a replacement photoshop, the open source community would already be profiting off the fact that a commercial photoshop already exists, and has years of product professionals spending large amounts of resources to understand and cater towards their users.
and if you're building on Gimp you're WAY past "from scratch". The bulk of it is already there. Mostly Gimp just needs usability / human interaction changes.
There is a neat feature of large fundraiser campaigns in free software you can take advantage of, in that if you do get 400k and hire 5 devs and have them work a year on a wide range of features for some software, it doesn't really matter what upstream says if the users want those features. You just fork if upstream isn't "on board", work on your own separate branch for a year, and then when all the users start downloading and using the fork the upstream will have to work with you to merge all the work done since 8000 hours of work on their project is not something they can discard.
As long as you wrote useful features then GIMP, for example, wouldn't be able to argue semantics on the patch set and tie up merging for a long time on politics. Their users would demand the features.
Surely the big problem with this (and the wider site concept, much as I generally favour it) is that the donating users who say they also want GIMP to be as good as Photoshop don't all have the same idea about what constitutes better UX. So you get a high proportion of dropouts from the funding subscription, as well as disagreements with GIMP's existing user base and team about whether they forget their original roadmap in favour of merging your patch, assuming you can still afford to finish it.
(Key difference with Kickstarter etc is that people are pledging for a specific solution rather that for people to solve a problem, and the cash is paid up front rather than a promised monthly commitment. And still there are issues with execution of many projects, particularly once third parties with different goals are involved. "I also need GIMP to be as Photoshop" on the other hand, is basically asking for a "nice unicorn" and hoping both that unicorns can exist and that everybody else involved - especially the unicorn-breeder - shares your definition of "nice" ones)
