Any project as durable and feature-laden as Thunderbird is bound to create or be facilitated by differences of opinion. The choices of paths to follow are many:
- pour more resources into Thunderbird,
- just maintain Thunderbird,
- develop a web-application that does the same things,
Mozilla has decided to freeze the features and concentrate on web/cloud stuff. That annoys some who have grown to depend on Thunderbird, particularly those with many e-mail accounts. Thunderbird makes sense for its ability to concentrate those accounts in one application.
That move may put pressure on the using community to contribute more to Thunderbird or the community may disperse to other clients or web-mail. We shall see. I would bet a FLOSS application like Thunderbird with tens of millions of users will live on in one form or another indefinitely and may well grow. I would not be surprised to see its functionality appear as a web application sooner or later.