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.
An android port is within top 3 requests as per Mozilla Connect's Thunderbird section. Makes sense to me to work on such requests when you're operating off donations.
Now you know and can get into voting for the suggestions you deem valuable!
I don't know if there are any other public surfaces the Thunderbird team uses to figure out what to work on (outside of bug reports/telemetry and Mozilla Connect).
May be it's just me, but I don't use email the same on my phone as I do desktop. I have 5 accounts on my desktop but just the one witch is my mane on my phone. Aps these days cant just stay good at one thing like ff being good on windows. They want to be good on Windows, Linux, BSD, Android so on and so on.
Sometimes I must open some stupid mail on my phone because an app needs it for verification or some other nonsense. I'd like to have an accessible option to do it.
Because it's a real time-waster to need to setup accounts on each device again and again, setup email signatures, setup canned responses, configure as needed etc. Apple has been able to sync accounts and signatures since the original iPhone with their Mail app.