It takes years and year to develop good defensemen. Hardest position to play in the NHL. Look at Darnell Nurse, he is a talent but he is still a work in progress. At some point, if the Leafs and Oilers truly want to win championships, they will open up their wallets and spend money on well established veteran defensemen. The Leafs are probably in a slightly better position to do so. Right now, the Leafs and Oilers are simply cheating on that side of the puck, and they aren't going to get away with it in the playoffs. This is not rocket science, it should be obvious to the respective fan bases.
I wholeheartedly agree with your statement. The Leafs are indeed in a better position to deal for a veteran top defenceman, but the problem is that they know they'll be in cap trouble a year or so from now when Auston Matthews, William Nylander and Mitchell Marner will all be due for a huge raise once their current first contract expires. Matthews' annual salary will likely jump up to $10 million/season. Who knows how much Nylander & Marner will get. There's talk about Drew Doughty wanting to join his hometown team once he becomes UFA after this season, but will the Leafs be able to pay him what he'll be asking? JVR will also be UFA after this season & i doubt the Leafs will agree to let him go for nothing. A sign-and-trade scenario could be the likely scenario.
This is why i think it'll be a HUGE mistake if the habs don't send young defenceman Victor Mete back to the London Knights. As you mentioned defenceman is the hardest position to learn in hockey. It often takes a longer time for defencemen to develop. Take Chris Chelios. When i watched him with the habs early on there's no way i could have predicted how good he'd become one day. If you would have told me back then that he'd one day win the Norris trophy i would have laughed!! But Mtl likely felt like i did since they wound up trading him to Chicago for an aging Denis Savard who's best years were clearly behind him at the time. And then Chelios became a star defenceman, headed to the Hockey Hall of Fame and would end up playing well into his forties.
If the habs keep Mete with the big team all season long they'll risk ruining him like they possibly ruined Alex Galchenyuk by keeping him with the big team at age 18 instead of sending him back to Sarnia for another year of junior hockey (he had also missed his entire previous year due to major reconstructive knee surgery).