Excel is a stretch he looked good and often the better part of the pairing with Nurse, but still the Expected Goals For for the duo was 30.2% it doesn't exactly scream a recipe for long term success.
Also, the mindset of well if you can do it in the most high pressure situation against the best competition surely you can repeat the feat in a lower pressure situation against weaker competition, is kind of a hollow one, the regular season represents its own set of unique challenges, for example when the games matter less does complacency creep into his game? How consistent can he be? Will he bounce back after a bad game or will it erode his confidence? When coaches more thoroughly scout him for weaknesses, can he make adjustments to potential weaknesses in his game?
The 2nd pairing is a high bar to reach for, I think it's a 50/50 chance he can hold that down to an adequate level, where as I'm more like 85-90% confident he can hold down the 3rd pairing and he is more likely to excel and grow his confidence in that role.
I also think people are running a little too high on the team after a SC Finals appearance and a good offseason of signings, there is a realistic chance that our 2nd & 3rd pairing (presuming Ceci or Kulak is traded) are below average to bad and if that occurs a single long term injury to any of McDavid, Drai, or Bouchard will harm the team enough that I wouldn't treat the playoffs as a foregone conclusion, we are still a team that is highly dependent on the straws that stir the drink despite the improved support around them.