Is this really a big issue? It's not like there have been any major changes in how email works. It's not like web browsing where things are constantly evolving. They wioll still be releasing security updates and bug fixes.
As Robert Kaye observed
: "When no one makes a decision that impedes progress. Sometime continuing work depends on someone making a decision which could affect downstream work. In order to avoid throwing work away, people will wait until a decision has been made." (This is true for just about any organization, not just OSS projects).
For Thunderbird, this means that if the leadership changes due to former developers only working maybe a few hours a week, the project hierarchy may get convoluted and thus the project may slow down or stop.
The hope [edit: my
hope] was that Mozilla would challenge Exchange and Lotus Notes and over time. It had been adding more and more features that indicated it might push into that territory and I much
prefer the interface to both of those clients. Without strong organizational and monetary backing, moving into corporate/enterprise territory is unlikely to happen. That said, the open source productivity tool LibreOffice seems to be doing pretty well independent of its former corporate sponsor so who knows? Hopefully Thunderbird will gain at least some financial support from elsewhere the way LibreOffice did.