Blues want to sign Pietrangelo and have already started clearing space (Jake Allen trade). They still need more space, can't trade Justin Faulk (NMC) and can't really afford/doesn't make sense to have 3 expensive RDs
Blues get 3 young cost-controlled players back.
It may seem like a low return, but remember 1) they need cap space; 2) they would otherwise let Pietrangelo go for free; 3) Parayko only has 2 years left which means he would need to be protected for expansion draft and then can go to UFA that next season
Open to other suggestions that are fair for Parayko (not Nylander cause we've already seen his value is low)
You might get his skate laces for that, how is his value low? because he would make the leafs better? STL doesnt listen if Nylander/Marner isnt involved