I would have done it for a 1st, nothing less....and I can't see why any team would value him that highly.
The cost on him was a 3rd, so what sense would it make to hurt your depth for this year, but also put yourself in a spot to need to give Williams a big second deal next year, and/or remove the flexibility of moving on from Collins?
My guess is someone offered what would be a low second....which would be roughly 20 or so players ahead of where McGovern was selected, so...Dallas would be hoping to what, replace him with that pick at best? No thanks.
If this FO knows what they are doing, and lately they very much seem to, McGovern is a starting guard here next year and the Collins/Williams savings is spent on the one year guys that are playing well...etc.