As far as I can tell, Valve are not the main source of contributions to Wine. Most of it is packaging Wine with other software in a user-friendly package. The only really major contribution they seem to have made is funding and helping the sole developer of DXVK.
The changes being submitted for consideration for use upstream are a licence requirement. Not that Valve don't want to help, but they don't have a choice if they want to use Wine.
I'm not saying Valve have done nothing, but I do think their contribution is grossly overblown. They also aren't the only game in town with Proton, with Lutris being the most notable other 'package' for running Windows programs.
Submitting changes upstream is not a license requirement, nothing in th GNU lesser license says that. Not sure where you are getting that from, I'm going to need a quote on the LGPL license agreement that enforces that. But you are completely wrong on that, Valve has 0 obligation to do that, the only obligation they have is to keep Proton in the LGPL license.
I never said they are the main source, I said they are ONE of the main contributors both financially and submissions, the main source is CodeWeavers which have been contracted by Valve since 2016 and also works on Proton, while also hosting and maintaining the Wine project and their commercial version of it, CrossOver. You can easily see that in the blog posts above.
Lutris isn't the same as Proton, Lutris is just a wizard, it then needs a runner and that runner is either Wine or, yes, Proton. I think you fundamentally don't know what Proton/Wine is or even does.