The point still stands though, if you have a mail provider that supports custom domains there usually isn't a limit on the catch all aliases you can add (or a very high one). In any case it's far from being "prohibitively" expensive.

> I'm also fairly sure that they don't let you reply as the address the catch-all was placed on

That is definitely possible with Fastmail as that's how I use it. In their case I believe that feature is called "Alias". If you reply to an email you just select with alias you want to reply from.