Bad scouting, drafting and management have been the biggest problem - totally outdated ways of handling things.
It is just not acceptable that you have McDavid and Draisaitl on your team for a bargain of $21M and are somehow not able to compete for the Stanley Cup annually.
Since 2012 the Oilers have had 16 draft picks in the first two rounds and there was no preeminent pick at all. McDavid and Draisaitl were self-explanatory, Nurse and Bouchard were good, but certainly not top tier picks. Skinner might be the only hit after all, time will tell.
You simply can't build a great team by only signing players because you will usually overpay on the market in order to acquire them.
Contracts are a disaster. Oilers paid Nurse (11p/20gp) and Campbell (.876) $14,25M. Simultaneously, Yzerman signed Husso (.922), Kubalik (21p/20gp) and Perron (16p/19gp) for $12M.
People argued that overpaying Nurse is no big deal, but it most definitely is if you could acquire a guy like Kubalik ($2,5M) for the money that Nurse and Skinner are above what they actually deserve.
Whereas the Oilers pick up some obsolete vets (mostly also on bad contracts).
This is all about bad analytics and scouting and decisions.
Now people will probably argue that Yzerman was very lucky and that the Oilers very unlucky. But what are the odds that some people can just repeat their success (Yzerman) over and over again whereas the Oilers just have bad luck through 10 years? Crap rolls downhill.
If any of McDrai decides to leave or if they won't have won anything by the end of their contracts, heads will have to roll. There is no way around. Not just for bad decision making for the Oilers, but by wasting the talent of some of the best players the game of hockey has ever produced.