The problem with aliases is that it's effectively a way to ensure vendor lock-in even for people with custom email domains. If one of the reasons to use a custom email domain is to preserve flexibility in choosing an email provider, good luck switching providers when you have hundreds of aliases set up in the hands of hundreds if not thousands of third-parties and your strategy for managing your inbox depends on these aliases continuing to function.

It may not be nearly as strong of a lock-in as a @gmail.com address, but I'd be curious to hear how many heavy alias users are paying Fastmail on a month-to-month basis.