I think the point is that to keep the project relevant the underling aging Gecko underpinnings may be going away. This would cause the maintainers to have to take on maintaining their dependencies as well as the core project. I can totally understand this as a motivating factor here, however, I use Thunderbird as a local offline mail client. Though I see these are out-of-fashion these days, it doesn't make the need for this kind of software go away. I only hope a web-stack version of thunderbird still stores settings and content locally and hopefully outside of the browser (not going to say electron here, but similar).