I think the biggest question with Schaefer is how quickly this rise has been. People have been talking about Hagens, Martone, and Misa as players in this range for years (certainly at least Hagens and Misa, and Martone has been talked about as top 5 for at least a year). These players have been under the spotlight for a long time, and have mostly done great everywhere they've played (can nitpick Misa from last year, for instance, but still on the whole we have years of data what these players are).
With Schaefer, he wasn't even being talked about as top 5 until the beginning of this season. That's partly because he wasn't playing at anywhere near this level last season. The beginning of last season people were questioning with Erie whether he was headed towards being an OHL bust. If you watched him play it was clear that wasn't going to happen, but other than the WHC17 last November where the competition was the same age and he wasn't playing on a bad team he struggled, relatively speaking, he wasn't playing anything like a top 2-3 pick. He got better as last season went on, but still not really the range of a top 2-3 pick. He was clearly outplayed by Hagens and Martone at the WJC18 last year.
He's not really went head to head with any of the top prospects at international tournaments, and outplayed them. If anything, it's been the opposite. It's questionable whether he's even outplayed Martone and Misa this season in the OHL. Even this season, he missed the beginning of it. It's been like two months for him playing this well as opposed to three or four for others. So there's a lack of a long track record. Unless you're just so sure he has the higher upside (and maybe that's the result come June, who knows?), it seems needlessly risky to consider him above these players.
Yes, he's younger than Hagens and Martone, and Misa had the extra year of junior, which inherently makes it a little easier. At the same time, just as people say it's an advantage to potentially have a little extra development runway by being younger, it's an advantage to have proven more. You cannot afford to get a top 5 pick wrong (especially 1OA). It's absolutely a relevant factor at 1 who has proven what for however long.
And even if you've come to the conclusion that he has such a higher ceiling than the others and it's worth it to take him despite the risks, we've seen over the years that it tends to be the wrong choice to take a defenseman 1OA (or at least to believe they've separated). In 2014, you have Ekblad at 1. You have Draisaitl 2 picks later. In 2018, Dahlin was viewed as this generational type of defenseman (not the generational that one poster is claiming Schaefer is) and on par with some of the best non-McDavid prospects of the last 10 years. We see with top 10 picks like Hughes, Tkachuk, Svechnikov, Bouchard from 2018, that he's maybe in the running for best player, but it's definitely not clear cut that he's another level. In 2021, it was thought that Power was so much better than everyone else (I'd argue he was viewed higher than Schaefer on consensus, FWIW), and now we see that it's a more complicated picture. He's not the clear cut best of the top picks, even if maybe he's still in contention for it. So it hasn't really materialized when people start suggesting they can evaluate a defenseman to project that much better than the other players in the draft.
Schaefer is a very good player. He's the shiny new toy, but I think I'd have Hagens and Martone over him right now, and maybe Misa. They've just proven that much more and the upside is at least comparable. Drafting for need at 1 is also a good way for a GM to get themselves fired. You rarely get the chance to pick first. It's negligent to take anything other than whoever you think is the best player. It's a lot easier to replace a regular old team need than to replace the best player in an NHL draft (what you hope you get with the 1OA).