In today's NHL you pay for the potential if you want to lock these kids up early. MacK will break out into a PPG+ player (I really don't say this about many players). His agent and the Avs both realize they have quite the player in MacK. In order to lock him up long term, it is going to cost a great deal of money, even if he only has a 65 point season.
Landy had a bad year before he signed, the Avs took a risk and paid for potential that all parties knew was there. They were rewarded with a contract that looks great now. Varly was rewarded with a big deal in the middle of his breakout season. You could argue he was still a risk there, and that if the Avs had waited until after the season, they are looking at a higher price tag because of his Vezina nomination. ROR had a great year coming off his ELC. The Avs had a chance to lock him up for what was though to be a bit much (here and around the league), and the Avs balked. We all know the end to that story means instead of having a $5m ROR for the next couple years, we are looking at a much different scenario. Barrie is another one that is going to cash in on his next contract. The Avs didn't pay up to get him signed long term last summer, and Barrie came out and cemented a huge deal with a big season.
At certain times, you take the risk and bet on the player. If there is anybody on the Avs to do that with, it is MacK. He is a sure thing.