> well, but I want it to be a focused app
Best of the both worlds can be achieved, for example, with good extension support. That way, Thunderbird devs can focus on a set of core features, while the community handles the rest.
Also, why not look Thunderbird as a Communication Software instead of just an Email Client?
Let's just face the fact here: email is dying. Once it was one of the most important tools for communicating on the Internet, now days for majority of people, it's just something comes with a Google/Microsoft/Apple account.
The protocols (POP3/SMTP/IMAP) are heavily outdated as well, and no one is seriously improving it in order to make them more fitting for today's needs (by improving, I mean to make it more real-time, more multi-media friendly, and more efficient so it can transmit a huge amount of data simultaneously at lighting speed). Just take look how many improvement has been done to HTTP as a comparison, it's sad that email protocols received so little loves.
At this circumstances, I would say any optimization done to the ecosystem is welcome. Also, I like the new interface more, IMO it makes Thunderbird at least 10 years younger.