http://bugs.winehq.org/show_bug.cgi?id=48006
--- Comment #61 from Andrew Church achurch+wine@achurch.org --- A Proton user reported (at https://github.com/ValveSoftware/Proton/issues/580#issuecomment-2821227806) that the new launcher can be prevented from installing Webview2 by creating an unwritable empty file at "Documents/My Games/FINAL FANTASY XIV - A Realm Reborn/MicrosoftEdgeWebview2Setup.exe", which then leads the launcher to offer the old layout. When I tried doing this, the launcher just froze on the splash screen without attempting to install Webview2, but after killing that process, I was able to access the old launcher by editing FFXIV_BOOT.cfg and changing BrowserType from 2 to 0, then making FFXIV_BOOT.cfg unwritable (because the launcher updater attempts to rewrite it to 2 before starting the launcher).
Presumably this BrowserType workaround will go away in the future, like the old one did, but it lets me successfully log into the game for the moment.
I can also confirm that with Wine 10.6, only the mutation hack is needed for the old launcher to work. Thanks to the Wine developers for getting the needed fixes in!