1) He is, he's looked pretty decent since his early-year AHL stint and callup. He's also only 21.
2) We've got a couple of good future #1 goalies already (Greaves and Ivanov), I'd be stoke d to have Silovs but not at the expense of Johnson, and it's not a huge position of need. Elvis is the now, Tarasov is the transition, and Greaves/Ivanov are the future.
3) We've got a future 3C already in one of Sillinger, Voronkov, and LDBB.
4) The first might as well be a 2nd, and Johnson hasn't been "bouncing between the NHL and AHL". He's a top prospect that got some needed seasoning in the AHL for a month to learn a new system and hasn't looked back since.

I think the trade itself isn't bad value, but it's a couple more darts with middle-to-high NHL potential (which we have a lot of) for an NHL regular and difference maker with what I'd call elite potential and a middle-6 floor. For Columbus, a team that's had a notoriously hard time developing and keeping those kinds of players, it makes more sense to keep Johnson.

Mitts had 60 points last year and his defensive game has vastly improved while his offensive game has grown. As AA said Pettersson is great but the trade is absurd and the Sabres don't need a 12 mill cap hit

For sure

Sure, 21 year old d+3 player with 122 NHL games already definitely needs to get a fresh start. Things are just going so terribly with him.