Strange that you use a still-popular, decentralized, open-standard global service with a myriad of paid and free providers, for which there exists dozens of open-source clients and servers across the planet, all of which integrate with each other... as a model for why Mastodon won't work.
Why do you say "gmail won"? Did gmail switch to a proprietary, closed protocol? Did they lose compatibility with even the smallest, independent email servers? (That was rhetorical-- provided those smaller servers adopt appropriate open, non-proprietary anti-spam measures (DMARC, SPF, DKIM, etc), they work fine.)
Meanwhile, no one uses AOL's old proprietary mail system, if it exists any more.
Why do you say "gmail won"? Did gmail switch to a proprietary, closed protocol? Did they lose compatibility with even the smallest, independent email servers? (That was rhetorical-- provided those smaller servers adopt appropriate open, non-proprietary anti-spam measures (DMARC, SPF, DKIM, etc), they work fine.)
Meanwhile, no one uses AOL's old proprietary mail system, if it exists any more.