No offense to Google, imap is inferior to both JAMP and Gmail's REST API for present day messaging and integration purposes. Google's IMAP implementation also has some...quirks.
It would be more accurate to compare JMAP to IMAP and SMTP, which have been around for forty years, and I think it's reasonable for me to calibrate forward looking expectations based on more recent protocols (such as JMAP). It would be wasteful to not consider increased utility from improved protocols currently in production for similar use cases.
Higher level, Google won't be around forever in its current form, so I think its reasonable to champion open standards and portability versus lock in to an org whose incentives between itself and its users is not that great. If you want to use a new Gmail version, great! If not, and Google doesn't offer ongoing support of a web version you're partial to, you can port out to another provider, just like a mobile phone provider.