Can you specifically say that 100% of the reason for this is the hotbar/combat system, or were there other factors involved? As a point, people who have invested time and effort in an MMO are less likely to leave said MMO or gravitate to a new one that is completely different in set up. Im less likely to jump over to EvE online coming from WoW because how they handle gameplay differently. Not that I couldnt, but would I be willing to go to a system that is drastically different and spend a ton of time relearning things. Part of the reason you have WoW players gravitating to FFXIV, is because of how similar they are in many ways. If you played Runescape and were used to a simple combat system, a complex system would be convoluted, especially if it was implemented poorly (not saying it was, but that may also be a factor.)
Alright, so Ill grant my initial wording wasnt clear. So Ill clarify it: Modern Mainstream MMOs do not function on 2 button combat systems, and would likely bleed players as a result. Mainstream is defined, as I previously said, by how relevant it is.
Now that Ive cleared that point up, let me point this out: Your position is using a niche (as it is one being an outlier of current MMO platforms, not the norm) Legacy MMO that isnt mainstream as an 'example' that an MMO could operate on that. Yes, as you prove, it could. But as I just clarified, modern Mainstream MMOs dont and implementing said system, particularly in FFXIV would end up killing it, not helping it. I predict MOST modern MMOs would end up dying off with that implementation.
The 2008 article point is in reference to you using a "Million subscriber count" as proof of it being more relevant than FFXIV by sub count. Which I was pointing out that if it hit 1 mil subs back in 2008, and it still is at 1 mil subs 2019 across all platforms as implied by the 2018 article, it would mean that runescape itself is most likely not a million subs. This gets more to the point that the more recent 2018 article does not differentiate between a few important factors, which I outlined, so its not a great example if were measuring by sub count alone.
No, its a theory as to why balancing down around easier mechanics at the average level probably end up damaging current MMOs. My original point when put into the context of Main Stream MMOs works just fine. Rather than trying to be "Gotcha!" about teh subject by using a legacy MMO as a prime example that isnt Mainstream nor has had much (if any) impact on games recently, you should try pulling apart what I was theorizing on by pointing out how if they made the game easier and easier around average players that it wouldnt hurt the MMO but would be a boon all the while keeping high end difficult content in game that gets progressively harder. Cause if you had to boil down your position, its pretty much : An old MMO thats still around has 2 button system and because of it, everything else you point out is irrelevant, regardless of the context of the thread at large, FFXIV, Mainstream MMOs and so forth.