The worst is that just changing the user agent makes everything work.
Are we back to the "built for Netscape 4.7" days? Sites shouldn't support browsers, but standards. By all means check if a browser supports a certain standard, and provide an error message for browsers that are too far behind, but don't ban them simply for having the wrong user agent.
It ends up being a feature requested by customer support teams that you prevent the app from working on unsupported browsers because users don’t read banners or pop ups and it generates pointless tickets. If you make your users change their UA there’s an awareness that things might break.