Thunderbird was on my mind when I was writing this. I think it ultimately comes down to whether they can afford any missteps or side concerns at all - and if the answer is "no", then Thunderbird must be cut loose no matter its value. It can always be mothballed until the times get better, or it can even be given "on loan" for some fixed duration to another trusted FOSS foundation and re-adopted when the time is up.
As good as Thunderbird is, I wouldn't want it to be anywhere near the top of Mozilla's priorities list right now.
I think Thunderbird gets to piggyback on some Mozilla infrastructure - hosting, CI/CD, receiving donations into a dedicated Thunderbird pool - but has been mostly cut out as you describe for some time. https://blog.thunderbird.net/2012/07/the-community-is-standi...
Thunderbird has always been their opportunity to demonstrate their stack is a general ecosystem, but it's headaches show that their stack isn't a good ecosystem for anything that isn't Firefox.
They shouldn't be trying to build thunderbird for its own sake, they should be demonstrating their equivalent for electron and feel pressured to make it no worse for users than the current Thunderbird, but attractive/stable enough for outside developers to choose over electron/etc.
For the record, I disagree with the assessment that they should be building an Electron email client. Most users are already using a paid/monetised Electron email client, a soon to become Electron client (like is supposedly planned for Microsoft Outlook), or a webmail interface.
Electron clients cannot compete on deep technically obscure feature sets, simply because they didn't have 20 years to accumulate them. They cannot compete on latency. Thunderbird should remain native.
Er, you do realise that Thunderbird isn't exactly native, either, what with it being based on XUL (or whatever's still left of that these days) and Gecko?
I think you are using dichotomies inappropriately to invoke the standard biases that have trapped the Mozilla community.
Something that competes with electron is not electron. Non-native includes RLBox which is now going to secure Firefox according to Mozilla. Mozilla is on record regretting thunderbird's poor Integration with the Firefox stack as a trial for thunderbird devs and a tax slowing Firefox engineering.
This holding pattern has gone on and on because no one wants to establish the correct API layer to maintain as an inherent tax for Firefox engineering that enables all F/OSS to reuse the NS* stack correctly with good documentation.
In the long term this means chromium has an ecosystem and Mozilla's stable ecosystem consists of just one browser. However flawed chromium is, it has no competition for most developers and competition with WebKit for a handful.
As good as Thunderbird is, I wouldn't want it to be anywhere near the top of Mozilla's priorities list right now.