Who is actually using JMAP? I never heard of that, what's the point of implementing something that is not used at all.
I think it is not a great idea to say that it is not a good idea to not implement things that no one uses, because that is a certain way to not make more people use it.
There are some servers that support it such as Stalwart [0], though I don't know about any older servers and/or clients that support it well.
(a) is IMAP good / sufficient / fit for purpose
(b) if not, is there a better alternative available
(c) if there is, is it worth the development effort to support it.
If we assume the answers to (a) & (b) are NO & JMAP, then the benefits are included in those answers & only question remaining is (c). The person I was replying to seemed to be only arguing that the answer to (c) is NO.
If this question made any sense we would never make anything new at all. How do you think new things get started?