There are still some hardcore MMOs out there (not all old), they just don't come with the money bag backing like big name games do. I love Souls games but I don't have the time to play and enjoy a game that is as challenging as Souls and as grindy as every MMO I know of (Souls games are not nearly as grindy as an MMO, and without making it something like a Sandbox you cannot "not" make it grindy). I think in a Sandbox MMO you might be able to find a happy balance of extreme hardcore specially since the content sort of comes about through play and is less based on strict railroading (more about you and other players actions). In a themepark I don't think many will have time to shout 3 hours to have a 4 hour party to kill 20 monsters and gain 2 levels - and that is why they are not as popular now.
That said I'm not necessarily suggesting themepark MMOs have to be easy, I think SE could have used a bit more intelligent* and stronger* monsters that you would need to approach with a lot more thought than "haha running through slyph lands, 20 monsters on my tail". *AI/Abilities, not straight buffs and more thoughtful abilities and counter strategies - like monsters that can run at your sprint speed and don't let down so soon, lunge and slow, the frog tongue grab in game is actually pretty good imo, monsters that use more deadly positional abilities, NM or NM like monsters breaking up expectations (greater level diversity of a set of monsters in an area - needing to pay attention to name and level disparity), the 'need' to stun more non-positional attacks (or be hurt greatly), making status effects seem a bit more serious or at least to match the theme of particular monsters (poison is weak, paralyze is actually pretty annoying on mage - imo that is good). Minor tweaks to make things a little bit scarier without making it impossible to go alone, though some monsters you would avoid or sincerely challenge.
Still difficulty but less expected I miss traveling forms (and related, more seamless areas loaded by grids instead of zone), and i don't mean 30 minute boat rides, not needed to be like WoW but as an example it had a number of .5-5 minute zeppelin/boats between islands and I just really liked the feeling that things had a scale that couldn't quite be trivialized and yet weren't incredibly annoying (or when you could teleport it felt powerful, mage portal to town was neat - nothing like FFXIV's teleporting feeling). Stopped around TBC though, so if it changed I'm not quite aware besides seeing occasional patch notes and videos.
So I guess tl;dr I agree a little bit but also I think in a themepark mmo that it is important to have tangible gain and plausible catch-up without having to skip every piece of side content, be a teen again (jobless, or unique job like that guy who says: 'I work at a server farm, and play games at work'), or be incredibly slow if you cant or are unwilling to sacrifice other parts of your real life (FFXIV is fairly easy to create tangible results without extreme amounts of time). I like that Yoshida follows something described like a rolling mountain.
Edit: Yes difficulty != grind, but if you keep them truly separate then you will still skew rewards greatly - there are a lot of good players and they would get these rewards nearly instantly and if repeatably then that too.. in an MMO grind is important and so regardless of higher difficulty you'd also need grind (now I'm also a huge fan of RNG that tracks failures so you don't become that super unlucky person as that isn't fun, but the grinding is important and imo inseparable in discussion of a themepark mmo).
From what Yoshida said, the overworld of 3.0 will be a bit more serious and that sounds nice. I'm curious how they went about that.