I guess the sad state of affair in regards to email syncing protocols (POP3 and IMAP) are partly to blame, JMAP might help a little but I guess it's too late.
I get email. I read them. I archive them. I only interact with each email for about 10s and then it's gone, usually forever. Very occasionally I need to search for something in the archive. I also need to send so I just need a text box to write in and somewhere to drop attachments.
What else is there that you need in a client?
Finding an attachment someone sent last week that you didn't have time to look at until now.
Finding a long set of instructions you sent to someone two years ago so you can forward it to someone else.
That they use to read your emails, harvest your data and advertise to you.
see https://notmuchmail.org/ your email archive just isn't that big
as for natural language processing, I'm not aware that Gmail does this for user search? maybe it does, but I haven't noticed more than some basic stemming. and given the choice (is not hidden), even most basic users prefer exact/stemmed keyword search over "cleverly" second-guessing returning noise results you didn't ask for.
also there's many non-proprietary search result sorting and filtering techniques that do a great (and predictable!) job, locally, without having to go all deeplearning about it.
remember that a lot of the sophisticated NLP analysis stuff Gmail does, is for the benefit of targeted advertising first and user experience second.
https://support.google.com/mail/answer/7190
Plus labels and filters allow me to automatically "tag" messages based on sender/domain/wildcards/other variables
I don't have a huge amount of emails but I manage 3 Gmail accounts using a single client (with inbox delegation) and also older Ymail and Hotmail accounts as POP3.
I'd say it's entirely that. Sync was so often half-broken with real email clients across multiple devices that having no offline access was a price nearly everyone was willing to pay to fix that, provided the web client were good enough (and, clearly, the problem was so bad that it didn't even need to be that good).
[EDIT] Also: accessing your email on devices you don't own/control.
I switching to running my own e-mail server around 2013 and have had a much better experience using just Thunderbird and occasionally RoundCube.
1. Search was better in Opera 12 than in any email client I've ever used (Gmail included). That has never been surpassed.
2. Gmail used to have excellent search, until recently. Recently I've observed many very glaring bugs with it, where swathes of matching emails are completely omitted from search results. This is something that may have gone unnoticed if I'd not begun to use the search function to regularly recall emails I knew existed, but it's become a repeated, almost reliably testable phenomenon with my own account.
+ 2+ minute wait for new emails to hit my inbox, + spinning wheel every time I load the inbox, and + oddly formatted html mail from coworkers.
Maybe those issues are to do with google's slow imap servers, but I finally realized no matter where the issues come from I need to get work done. That can't happen with the combination of gmail, large accounts, and any external client I know of.
And I have to say, after installing a keyboard plugin I can read email without using the mouse. Something I never got right with t-bird.