Not for BD - all 15 will play FC cricket, BPL and most of them will play for BD.
The reason is simply demand supply - half of the U19 players can’t make it to FC level for established cricket countries simply because there are already settled squads; you simply can’t drop a 26 years old seasoned performer to accommodate a 19 years old kid just because the kid has played an U19 WC.
Take India - Unmukt Chand & Ashok Maneria led their U19 team (Mandeep Singh as well); long back in 1990s there were Jatin Paranjappe, Amol Majumdar, Amit Pagnis .... Chand, Zol & Maneria are still active in cricket, but even in their late 20s, couldn’t fix their FC spot - by the time the senior batch are phased out, next batch of U19 boys are there. AUS won 1988 edition, and only few of them played FC cricket, and Holdsworth played one ODI - the reason isn’t those boys were not good enough, rather those days when they were ready for FC debut (say 1990 season), six Shield teams were stringers than many Test teams, and NSW could have competed with Lloyd’s WIN ball by ball. It was the same case for PAK as well in past because of the talent pool already settled.
For BD boys, most of them are already better than the average FC cricketers there, few probably better than national players. Previously, between 2002 to 2006, we had three very good U19 teams - half of them went to BD colours directly from U19 cricket - Ashraful, Omer Belim, Mash, Shakib, Tammim, Mushi, Mahmudullah, Enamul, Rubel... this batch won’t be that lucky, but each of them will make the highest domestic level and many will eventually represent BD.