Yes, playoff teams should give up good prospects for rentals, unless they're all managed by Bergevin who wanted to collect every Tinordi and every Scherbak. They all need to improve if they want the brass ring lest they become perpetual first and second round also -rans.
How many prospects and draft picks have the Vegas Golden Knights given up in the past few years? And they didn't all work out. You don't know which will work out in advance.
Pittsburgh gave up *a lot* of players and draft picks over the years. They didn't all work out, but in the end they won three Cups.
Los Angeles, prior to their dynasty, gave up Brayden Schenn, Wayne Simmonds, Jack Johnson, a 1st, Colton Teubert, and probably other players I forget about.
How much has Tampa Bay recently given up?
Did Colorado not give up Justin Barron for a guy who was a third liner on an awful team?
If playoff teams are not interested in any of Savard, Monahan, Allen, Harris, Kovacevic, etc it plausibly comes down to their GMs not being serious about winning.