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Changing the UIs within this particular family of software products has often given questionable results, confusing or upsetting the core user base for little real benefit. Other UIs built on JS and HTML technologies have so far tended to be slow and buggy compared to using native technologies on the desktop and mobile platforms. If the exercise is expected to take as long as 3 years with the available resources and the hope of getting back to more of a community-supported culture, that seems like a huge amount of work to essentially stay where you already were.
Short answer: security bugs. Gecko has too high of a code churn to maintain long-term support branches, particularly for people who have little knowledge of the components in question (e.g., SpiderMonkey). Forking Gecko might be a viable short-term strategy, but it's not at all viable long-term.
There's a lot of low-level interfaces and design decisions that filter through Thunderbird, for example networking interfaces. Once you talk about moving to a different rendering engine for the UI, you're more or less talking about ripping out ¾ of the backend code or so anyways.
Honestly, if I could use it without a Nylas account I would very seriously consider it as a Thunderbird replacement.
From reading the original email thread it also looks like the Thunderbird guys are paying attention to Nylas.
I do not want my mail client or any other app that should be always running in the background to be written in Electron.
If Thunderbird isn't updated, it will slowly die of bit rot.
