I agree, how could the Oilers not be considered a dynasty? In 1985-86 they were first in the league with 119 points, won 56 games, and outscored the next closest team by 72 goals. They were clearly the best team in the league. Yet because Calgary pulled off one of the flukiest and biggest upsets of all-time (Steve Smith own goal in game 7), you look at their 5 Cups in 7 years, 3 President's Trophies in 4 years, and 6 straight years of leading the league in goalscoring and don't see a dynasty?
Streaks are overrated anyway. Both the Islanders and the Oilers won 4 Cups in 5 years. To me, the fact that the Islanders' loss came at the end of their run and the Oilers' loss came in the middle makes no difference at all.
The reality is also that the playoffs are a crapshoot. The best team usually doesn't win, because there are many things like luck, slumps, injuries, bad calls, hot goalies, etc. that get in the way. Even if there is a super dominant team with a 90% chance of winning every playoff series they play in, the chance of them winning 3 Cups in a row is slightly better than 1 in 4. This shows just how special those Edmonton and New York Islanders teams were.
If you want to keep using Original Six standards in your definition of dynasties then go ahead and do so, but you'll probably never see another one in your lifetime.