Slack does not limit the number of people who can sign up for it. If Wave gave everyone using it infinite invites then it could have worked fine, but they did not.

Was Wave part of gapps though? Or did it predate all of that? Maybe it might have had a chance if you could have collaborated within an organization rather than only as individuals.

Edit to answer my own question, looks like the original G Suite was in August 2006, but at the time it was just GMail, Calendar, and Sites. Docs was added in early 2007. Wave was a thing under Google from 2009-2012, but was never part of G Suite.