He played them on the same line because Drai couldn't run his own line due to being severely injured, he could barely move in the latter half of the Dallas series and beyond so he needed to be sheltered.
I don't know why you're not acknowledging that the depth didn't score enough.
Here are the goals in that series from non-McDrai lines...
Game 3 - Broberg, Foegele, McLeod (and btw, McDrai had 0 goals and 3 points combined in the first 3 games)
Game 4 - Holloway scored twice, Janmark, Henrique, McLeod and Nurse all scored.
Game 5 - That was a McDavid game but Brown also scored and it was a big one to set the tone in that game
Game 6 - Henrique, Foegele, McLeod, Nurse
Game 7 - Janmark (only goal of the game)
That's a lot of depth guys on the scoresheet. The Oilers lost that series, and the Cup, because they got hardly anything from their top guys in the losses and they were outgoalied in a few of the games with Game 7 being the most notable one.
Even in the playoffs as a whole, Henrique had 4 goals, 7 points and was +3
Janmark had 4 goals, 8 points and was a +6
Brown had 2 goals, 6 points in 19 games and was a +4.
All huge parts of a historically good PK as well
McLeod was poor especially in the Florida series but he scored some goals, Holloway was good, Foegele was really good in the Florida series.
The depth did their part and were especially instrumental in the last two series.