The cap is going to be very tight, an extra million in dead space will be important. They're going into the offseason with $74 million on the cap, needing to re-sign Engvall, Sandin, Liljegren, and probably Giordano, and then fill out the 4th line and 7th D with league minimum types, at minimum.
Buying out Mrazek gives you $2.8M in extra space next season, but they would still need to sign two goalies (or find a legit starter + Kallgren).
Moving Holl gives you another $2 million, but you need to replace him with at least a warm body to sit in the press box, so that'll only save you around a million.
And again, I'm just not super high on Robertson anymore (I used to be). If the price to move Mrazek only ends up being Robertson + mid-round pick, that is a big win.