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.
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.
If Thunderbird isn't updated, it will slowly die of bit rot.
