I hate to bring up Gagner again... but he's a decent example. After 7 years in the NHL, he really didn't develop a lot more physicality, defensive awareness, faceoff ability, skating speed etc.
No doubt Sam improved somewhat (marginally) in each of those areas from his 18 year old season BUT markedly so? I couldn't really point to results on the ice that would demonstrate he benefited from years of development.
Now some will say Gagner was rushed and that ruined his development and set him back permanently and an extra year or two in Junior would have benefited him.
That's possible to a degree but I honestly don't see how that would have made him, stronger, faster, better defensively, better on faceoffs playing vs 17/18/19 year old junior competition for another year or two.
It may have helped his confidence to come into the NHL as a 19/20 year old with one or two more dominating Junior years under his belt... but I think the realities of the NHL game... faster, more physical, and more demanding of solid defensive awareness especially at the centre position... would have hit Gagner just the same as a 20 year old rookie as it did as an 18 year old.
Long story short for me... if a player "has it"... they will make it. If they don't they won't.
Top 3 picks in this era are usually players that have been analyzed and scrutinized to the nth degree. Literally hundreds of scouts are assessing these players and their weaknesses and strengths are fairly widely known. These top ranked prospects are probably better prepared than ever before as well... modern training/conditioning/strength/skating programs are used at the junior levels and in the off-season and specifically in the case of Draisaitl his father is an experienced professional hockey player AND coach. He's no doubt very serious and dedicated to doing things the right way and not naive or underestimating the challenges ahead of him.
No doubt Draisaitl will have his struggles and issues as a rookie but I personally don't view it as a massive mistake or damaging if he's allowed to play this season with the Oilers instead of going back to Juniors.