The nice thing is that once things are setup in Emacs, they usually work without much maintenance or surprises for years to come. How many times have mainstream email clients changed UIs and featuresets in the last 10 years? How many of them do you expect to still exist 10 years from now?
Another benefit is that using standard mailbox formats and separate tools allows you to configure, replace, and integrate any part of the setup. With traditional clients you're locked into whatever they support and allow you to configure.
And yet: my mail flow on my Mac, using the native client, is effectively unchanged for 20 years. I've changed back-ends, sure, but that's a different thing.
Whatever works for you, but I don't believe that your mail flow on the Mac is "effectively unchanged for 20 years".
Wikipedia lists many UI and functionality changes over the years for the Mail app.[1] As recently as this year, people are upset about changes in the Mail app in recent versions.[2]
Another benefit is that using standard mailbox formats and separate tools allows you to configure, replace, and integrate any part of the setup. With traditional clients you're locked into whatever they support and allow you to configure.