Thunderbird is absolutely not niche.

Gmail, Hotmail, Yahoo Mail are email services first and foremost. Most native clients do not have mail in the name.

At this point, probably most (non-mobile) native clients other Outlook probably can be considered niche. For consumer email, webmail has won.

We could quibble over how we're defining niche, but looking to our less technical friends that still use a variety of consumer & enterprise tech products/services daily is quite clarifying. My close friends, whether they work @ Impossible Foods on supply chain, @ a CA high school teaching history (now remotely), or @ SMB SAAS company (Hubspot competitor, forgetting name) HAVE NO IDEA WHAT THUNDERBIRD IS.

And they likely think that the computer monitor is the actual computer.

Is Yahoo Mail or Apple Mail really that different to Foobar Matrix Client (shortened to Foobar)?

Irrelevant to what I said. We talked about most products.