I've actually cooled on XMPP due to how poorly it often handles network transitions and temporary dropouts making it kindof a terrible experience on mobile devices, but yeah, I'd certainly rather use XMPP than Slack. Perhaps a better Slack alternative is Matrix/Riot.im, which has most of the upsides that Slack does but is federated and entirely FOSS (it still has an IRC bridge, too, unlike Slack).