The announcement from Mitchell Baker really is corporate drivel at its worst: https://groups.google.com/d/msg/mozilla.governance/kAyVlhfEc... This also seems like a shame. I'm not sure Thunderbird was ever a great product, but it was always a decent alternative to worse things, and I remember Enigmail being quite a good option as a relatively simple way of signing and encrypting emails.
Open Source may be well and good but the engineers at Mozilla still have to earn money for their families. Look at the Corporation's Financial Statements (https://static.mozilla.com/moco/en-US/pdf/Mozilla_Audited_Fi...) - they don't exactly show a company flush with cash that is pulling support for a product because they don't like it or something.
I like Enigmail though, just as you do, and tried introducing my family to it. Sadly that didn't stick :/