Why does there have to be a Thunderbird at all? The people building Thunderbird are clearly motivated to build FOSS/Libre communication tools for people.

As more and more people move to using Android and ios as their primary computing environment, the existence of accessible FOSS/Libre tools such as Thunderbird becomes more important than ever.

So, if they're interested or dedicated to building open tools for people to use, they can choose where to go, whether you like it or not. :)

e: By the way, have you donated to Thunderbird? If you are able to pay for the software you use, especially when it seeks to be a force for good, then you should. Even $20 a year goes a long way for an organization like this.