What actually do people expect from an email client that they don't get from a web-based one?
Managing multiple email accounts in a single interface.
Easily moving emails between different accounts and from online to offline.
Relatedly, storing years of emails with attachments on a local drive is cheap. Storing them in webmail is a hefty subscription fee. I have no issue with one time payments, but I like to minimize subscriptions.
My only explanation is that local search is inherently cross-team and integration-heavy. Consequently, if there's no higher-org prioritization it just molds and breaks over time, as unaddressed integration bugs pile up that cannot be fixed by a single team.
Companies shipping their dysfunctional org chart.
Still, I never thought I'd say that I could do things with the built-in search 20 years ago that I can't today.
Having that in one place, is essential.