Still better than what any evaluator would get at random positions in the lottery for those drafts.
Well, here's a little chart I just whipped up.
Top 10 picks over a period of a decade and their BPM. I chose to start in 2020 because it's unfair to take players with fewer than 4 seasons under their belt and compare them to seasoned vets. I chose BPM because I like the stat and because it's not cumulative, so again, it doesn't reward people who've played more games.
Remember that BPM is a stat that tells you how many points a player contributes compared to a league average player on a league average team. BPM of 0 would make you a very average player in the league. It's not a high bar at all. I've highlighted all the players who do have a positive BPM and then I've singled out ones with one higher than 3. The average for an All-Star in the season they were selected to the game has been just over 5 the past half a decade so it seemed like a somewhat relevant number. Those would be our studs, the sort of players you're hoping to draft with a top 10(or as this chart would indicate, probably top 5, pick).
Now, keep in mind that averages aren't everything. Both Zion and Simmons are coloured red on this chart and Simmons is definitely a bust and not even close to a stud. Both Zion and Embiid are also getting close to a point where you have to say that while they're amazing when they're on the floor, they just can't seem to be on the floor when it matters. Still, it helps to pick early, as you've mentioned, but it's far from a sure thing. If anything, this chart would suggest that picking 3rd might give you a better chance than picking 1st and it definitely gives you a better one than picking second.
As far as the rest of the draft goes, here are the +3 BPM studs picked outside of top 10 in these years.
2020: Haliburton
2019: none
2018: SGA
2017: Mitchell
2016: Sabonis
2015: none
2014: Jokić
2013: Giannis, Gobert
2012: none
2011: Leonard, Butler
9 studs 11-60 versus 9 studs(I can't take Simmons seriously here) picked 1-10.
Yes, it helps to pick top 5, but the difference is not as huge as it might seem. NBA teams have objectively gotten worse at drafting and there are more busts now than ever, especially at the top. Do you wanna pick top 5 instead of not picking top 5? Of course, of course. It's just that the cost of it may be greater than it seems.