kirbyThe second purpose is denying them health, so if the medic takes a lot of damage from a team mate, then you can hopefully finish the job. This works the other way around too - if you don't drop the medic but damage him enough for some spam to do the job, then have your team collapse on them without endangering your medic.
The third purpose is being able to wait for the medic and drop him once he's between the entrance and choke and banana/elbow/whatever. The one way this can be countered is if 1) scouts getting the call that you're up there, but they're easy to deal with. If they're being a problem, then go down drop down, wait a couple seconds, jump back up. Take the health kit again for more health and then drop the medic if you can. 2) Combo getting the call and knowing you're still behind them and constantly keeping tabs on you. Instead of dropping the medic then, distract for your team.
You're relying on getting the demo kill for both these to come into play, and that demo kill is far from guaranteed. You're relying on waaaay too much to go as planned.
It's a gimmick rollout that might win you one or two mids per scrim/match. Fast gunboat routes to mid are suicide unless you're playing some seriously garbage scouts and a slow ass demo
Also, you're not denying the enemy team any health other than that initial demo pack, assuming you get that kill AND deny him the pack... the pack is getting taken either way how are you denying their team the pack??? Unless you're just chilling in that choke the whole mid, in which case you already won the mid for your team with the demo pick and they probably aren't committing their med
Third purpose... "you already won the mid for your team with the demo pick and they probably aren't committing their med"
The whole point of this kind of shit is to insta-win mid by surprising and gaying the fuck out of the demo, at which point your follow up strats are useless. If you die w/o getting that demo, your team is 5v6 before your combo can even get there