But as far as an individual app developer choosing only to support Gmail, or only support whatever. That's their choice. That's not supplanting an industry wide standard. The case of Mailbox is very different from say, Mozilla removing IMAP support from Thunderbird, or Apple removing IMAP support from Mail.app. Mailbox as designed from the beginning as a Gmail client. That's not a flaw in the design, it's the intended design.
If you want a more general purpose mail client, make one. Don't fault other developers who weren't intending on making that in the first place.