Don’t sell this idea short! I would absolutely use a Windows GUI installer for juliaup if it were an option on the official JuliaLang install page. And I’m not alone it seems…
So there are three people just from this thread! 
As @lmiq mentioned, the only “problem” is be that you would (at least initially) be responsible for maintaining the installer and would need to consider if you want to make that time commitment. But, would people use it? For sure. 