Re Johnson: It was the Covid scouting year in 2021, many prospects especially in the CHL played less than full seasons and as in Cossa's case weird short seasons vs. limited number of teams. Aside from Wright, the top drafted former CHL guys McTavish and Clarke, played in Europe. Sillinger moved from the WHL to the USHL. The result was that few CHL-based players were picked in the top half of the Draft, NHL teams trusting more their evaluations of prospects who played more.
Johnston didn't come out of nowhere, he's always been considered a very good player. He was the 6th OA pick in the OHL Draft, played well at the Hockey Canada U17 tournament, was selected to the camp for the Hlinka Gretzky tournament which got cancelled, Dallas obviously liked him, made sure they got a close look at him during his inactive Draft season, had an opportunity at the U18 to gauge progress vs. other good players. Did they think he'd be this good? Probably not but you can't say for certain they didn't think he might be. He could have been top 10 on their draft list, the limited scouting opportunity for many prospects inevitably leading to even more differences in team boards than normal.
There is some degree of luck with almost every successful pick beyond the obvious top players, even if it's just luck that he was still available. In Johnston's case it may have been fortunate for Dallas he didn't play at all until the U18, and when he did it was on a stacked team of guys who'd played at least a little bit that season and may have stood out more simply because of that. But it's more likely someone on Dallas always believed in him and has been proved correct, not lucky.