In practical perl, multiple inheritance is used, composition is used, delegation is used, roles are used. None of these concepts or their usages are going away.
The Corinna MVP is designed to support composition, delegation, and roles. It almost sounds like you're suggesting we're taking this away, but we're not, so I must be misunderstanding what you're saying. And if you absolutely need multiple inheritance, we have a special syntax for this very edge case.
Corinna's not taking anything away. It's simply that the MVP is designed to be cautious by nature rather than risk all the missteps Perl has had with given/when, smartmatch, pseudohashes, indirect object syntax, the SUPER bug, and so on. I'm not saying we haven't made mistakes, but rather than shove too many things into Corinna precipitously, we need to understand how the MVP will play out.
After we work out the kinks in the MVP (and we will have them), if you want native MI support in Corinna, lobby for it and start and RFC. You don't even have to implement it yourself. Win enough people to your position and convince someone on P5P to sponsor the work.