But fundamentally the "folder" view of email does not work. A single message often needs to be in several different folders simultaneously. And when the thread is spread across many folders, there needs to be a way to see the whole thread.
The only way to accomplish this is with email tags or labels. These are implemented by nearly all successful email companies. Gmail, Fastmail, and Proton are examples. Labels are a fundamental feature in this day and age, and neither IMAP nor POP can handle them gracefully.
Gmail is so big that when Outlook, Apple Mail, and even Thunderbird connect to it, they do an OAuth exchange and then talk over a proprietary protocol.
JMAP may have poor adoption, but it's the only open protocol that understands labels well. The lack of adoption is mostly due to email providers not implementing it. There is not a lot of incentive for clients to implement it for the few providers. And providers would prefer you use their web clients anyway, as then they control access to your email.