I'd counter with a few things here..
Aside from Bjelica, none of those guys can we get. But, that said, Bjelica is a legitimate upgrade because of size, because he's a legitimate rebounder, and because he can certainly be relied upon to score much more so than Niang. The Morris twins are primarily for size and defense and a body to throw at bigger wings/4s. Bertans would have some serious gravity. I know the argument is that Niang shoots an equivalent precentage, but Bertans is a guy who can shoot over guys and will light it up from absolutely anywhere on the floor, and he gets that off quick. Niang is good, but he's not Korver good. Bertans may be Korver good. And his 10.4 three point attempts per 36 may be a new record.
But I think we're being lulled into passivity here. We've had an easy stretch and there will come a point where we'll want to sell low again. Rolling into the playoffs with Bradley and Niang as your backup frontcourt is a problem. We'll look back on this and think "well they were playing well and we were winning so the decision to stick with it made sense at the time." At the very minimum we need an Udoh.