The primary problem I see with your argument, and the examples of what 'main story' combat should be like, is that, while single-player games can afford to be as difficult as possible from the start, working their way down in difficulty via 'cheats' or 'easy modes' for the player, MMORPGs cannot.
The primary goal of an MMORPG is player retention.
MMORPGs do provide increasing challenges to players over time, even in the main campaign. I certainly found that to be so while playing this game across nearly 8 years.
A problem rises, however, when players who are used to the so-called 'difficult' single-user video games are disappointed in the level of challenge in an MMORPG.
In this game, there is both Savage and Ultimate mode (and whatever level of challenge can be found in Criterion dungeon instances). If they are not "hard" enough to satisfy your tastes, it is far better to either accept the fact that this game will never be as hard as one of the single-player games you enjoy or look for an MMORPG (or single-player game) that will satisfy.
Unlike a number of single-player games, this main story is designed to be completed, not to thwart players attempts at success.
You appear to be extremely confident that a majority of players would stick around if main story encounters reached Normal-mode 8-person Raid (or higher) levels of difficulty.While some players may feel left behind by such a decision, I do think it would be in the interest of the majority to try and find a better balance than what we’ve been seeing nowadays.
The bottom line for the company is to keep those players who "may feel left behind" subscribed. The difficulty of the main story line is geared to that retention. You may not like it, but companies depend on players with multiple levels of video game experience.
Since this seems to be the first MMO for a large number of those players, one would be foolish to ignore the possibility that "harder story line = fewer subscribers = lower income".
I can guarantee that "harder story line = more subscribers = higher income" is not a possibility in the video game MMORPG industry, or someone would have done exactly that at some point in the last two decades. [Rest in Peace, Wildstar].