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.

Right, but isn't the argument quite a few people are making here that moving to a different UI rendering engine doesn't seem to be obviously justified, given the goals of the project and its current situation? If the architecture is as inter-dependent and non-modular as your comments suggest, wouldn't that make changing significant parts of it even less likely to be successful?

At that kind of level, you might be better putting the whole project into legacy mode with emergency updates only, and just letting it carry on for as long as it's useful and then die of natural causes when the time is right. For the other goals mentioned in the original proposal we were talking about, if there's sufficient interest to support doing that much work then an entirely new project with no baggage at all seems like a safer and more realistic bet.

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.