The fact that FFXIV exists and is as successful as it is disagrees with this statement entirely. SW:ToR is another example of how an MMO can do story well.
I agree that the older content is sometimes difficult to get a party for, or just a long time in Duty Finder queue, but that's not really a testament to how story doesn't belong in an MMO, it's the same in any other MMO with level appropriate dungeons where the lower level ones just take longer to find parties for. I've done the slog from 1-50 4 times (and the ARR to HW 3 times) and haven't ever had a major problem getting parties through the duty finder. The longest wait times I had were for Castrum Meridianum and Preatorium as a DPS, and the waits then were ~40-50 minutes...which yes is excessive but were also the exception, not the rule, everything else took the normal 10-20 minutes.
So while I can agree with the sentiment that it isn't friendly to players returning to the game, I just can't agree with that in truth because for players that love and enjoy the story, it's not "unfriendly" it's part of why they're playing this game in the first place.