It's all about cost/benefit analysis. Maintaining an e-mail server and not getting overwhelmed by spam is hard; making sure you your e-mails will be received is even harder. Most of small businesses or organizations simply don't have resources to do that, if you count in the value lost when business e-mails get flagged as spam.

Decentralization is cool and all, but until we have some kind of self-contained, easy to deploy and maintain solution for e-mail servers, it's hard to me to recommend anything other than GMail to people - especially that typical small company or organization has a lifespan that's shorter than Google, so they don't get any important benefit from decentralization.