Nothing really stops them to keep changing stuff next Monday, or the following after.All the balance changes should've been set in stone on the final week of PTS and allow that week to be used to catch showstopper bugs.
I think it's perfectly fine for them to use the PTS to find the overperforming combinations and then make adjustments to compensate. But as the OP already said, it's a timing issue. For example, they should've nerfed Mother's Sorrow in last Monday's PTS, not on the Monday of Live. Esp. since it was abundantly clear last by Monday that MS was overperforming.