The problem with that is, using readline 7 as an example, you need to have had readline 7's image cached on your machine, or it still needs to be 'tappable' or whatever in order to do that. In this case, brew master _only_ had readline 8 available, so if you never had readline 7 installed on your machine, it was inaccessible without bothersome workarounds.

In most cases, brew just works. That's great. But it's also used primarily by developers, and I can't imagine this is an uncommon scenario.