How does Windows 7 preserve compatibility with older games? Dolphin's goal is to be compatible with all Gamecube (and Wii) games. If Windows 7 is impeding that goal, I don't see how Windows 7 will have better compatibility with older gamecube game.
Also, from the perspective of Windows 7, every Gamecube game had already been released by the time Windows 7 was generally available (the Gamecube's EOL was in 2007; Windows 7 came out in 2010). No game could have "targeted" windows 7.
At this point - that's the users problem. Don't say you don't want to engage with the developers of your software and then be flabbergasted when they stop supporting you.
I personally have a fork of Dolphin that retains compatibility with OS X 10.9 Mavericks. The hardest part was building QT 5.9 with Mavericks compatibility.
I don't doubt that Windows 7 usage is dwindling, but it's largely a self-fulfilling prophecy as developers miscount the size of their legacy user base which makes the application break on older systems and then the users either stop updating which means developers see fewer users in their metrics. It's unfortunate that the Dolphin team couldn't see the value in continuing legacy support and even more unfortunate that it's Qt's fault they had to abandon it.