I hear you on the verbosity of generic type declarations. It's something that I pushed hard for us to improve in the early phases of the project, but I was outvoted by other members of my team. But... at this point those members have all left the team and in the meantime we've heart actual users complain about the verbosity (not our own hypothetical "what if" complaints in the design phase) so I'm optimistic we'll be able to reduce generic type verbosity in the future. For example, a while back I did a prototype/experiment to drastically drop the verbosity of generic type annotations[1]. It's definitely on our radar.
Happy to chat more either on Sorbet's issue tracker or Slack group.