> people prefer Discord to IRC ... prefer Slack to SMTP or Jabber
"Centralization wins" is the wrong conclusion from this dataset. Discord & Slack have seen far more investment in UX & backend than IRC & Jabber. Because chat is addictive, relatively easy to implement, and easy for users to access via website or app, there's huge incentive roll your own chat service, and then incentive to differentiate from innumerable competitors. Many users are on half a dozen chat sites.
Email is going strong, even tho it's seen rather little UX investment. Decentralization is one feature keeping it afloat in a world of uncountable chat alternatives.