The Oilers are never going to have an elite D and goaltending. I don't think they ever did in their history really, the Oilers have been built from forward on down with a few exceptions so with this being the case, the Oilers need strength in numbers up front so when they're not getting contributions from support players there's always an incredible burden placed on two players to produce all the time. Their GA is actually pretty decent in the regular season but it seems to crater against playoff teams so they need the offense to outscore it and they just get enough from enough people.
Guys like Ryan and Janmark are such a tiny symptom of a much bigger issue and that is RNH, Kane and McLeod. You can't have a top line winger go scoreless for this long playing with generational talent. RNH pulls his "just sort of there" act way too often in a position where he should be scoring goals, he doesn't use the middle of the ice and he doesn't bear down on gifts from McDavid. Kane hasn't scored a goal since the Lincoln administration, you can't have a top 6/9 winger go absent for that long. McLeod provides nothing when not playing with Draisaitl this season, same with Foegele really.
You can have a 4th line that can't score as long as they're defensively sound and your 3rd line can score. The Oilers have a 3rd AND 4th line that can't score which isn't going to cut it come playoff time. They've kept GA down at a decent rate but you're not going to win many playoff series with them throwing up blanks on the scoresheet most of the time, depth is usually the decider in playoff series and the Oilers come up short in that regard even though they usually have the advantage with the top line.
The old and slow thing has been an issue all season as they've somehow found a way to get older and slower as the season has wore on but that's a whole other wall of text for another post