Are there really that many of them? Let's say he has the tools to get to 20 ppg by his third year or so. The NBA had 31 players at that tier this season, an average of basically one per team. That's not exactly a surplus. Getting that from a big or a point/combo guard is different from getting it from a wing. Looking down the list, here are the legitimate wings that I see (in descending order by scoring average):
DeRozan
James
Leonard
Durant
Butler
George
Wiggins
Antetokounmpo
Anthony
Thompson
Booker
Hayward
If one believes that Tatum's overall game could develop to the level of any of those players above, I don't see the hesitation in taking him at #4.
He said "can't do anything else". Most of the guys you mentioned who got 20 ppg contributed besides just scoring.
Leonard - Elite defender & rebounder
Durant- Elite rebounder & good defender
Butler - Elite defender & decent playmaker
George - Elite defender & decent rebounder
James - Elite at everything
Wiggins - You're right here
Giannis - Elite at everything
Anthony - Used to be good rebounder
Thompson - Elite defender
DeRozan - You're right here
Hayward - Great playmaker, solid defender