Why do MMOs these days don't offer amazing moments or hear extensive big praise any longer?
Play experience has been so incredibly homogenized except for the crazy extremes (and if you're not already "in" with those extremes, you never will be). Even if you only played WoW for its first year of release, you can almost certainly jump into any modern MMO and understand exactly what to do with almost zero adjustment period.
Is the genre dying out or are we all just jaded and nostalgic for that one game we see as perfect?
On the "dying off" half of the question...
First off, publishers are scared to try new things. That's a general rule and obvious assumption to follow with game development. It makes sense as new things are risky and video game development is insanely expensive. They want to recoup that cost and make a profit. If the publisher doesn't believe a project will fulfill that expectation, they won't fund it. Call of Duty, being crazy derivative and criticized as such, still makes money at the end of the day. As a developer, you end up with a shitty situation, stuck between two demands that can be counterproductive. On one side,
Secondly, publishers are burnt on the MMO genre. There have been...a LOT of MMO failures in the last decade. It's kind of insane how many failed MMOs there really are when you start looking. And for the most part, the same games are still on the top of the pile: WoW, EVE, some random ass Korean and Chinese MMOs that I can't be bothered to look up but I do know exist as top games, and a few others (it's actually kind of difficult to find real data on this that isn't being erroneously propagated by "journalists" that include games with notoriously low populations or do things like list fucking Minecraft as an MMORPG). The only real newcomer with any real weight and sticking power is FFXIV and honestly, you can jump from WoW to FFXIV and almost feel no transition between the two. So what lesson does this provide? Well, the only real contenders in the MMO genre that have REALLY stuck around are basically WoW, a game similar to WoW that has an established household name being "Final Fantasy", and a game that no one will ever manage to unseat as the king of pretty spreadsheets. Publishers have wised up and are backing away from the MMO genre as a result.
Finally? "Free to play." The alleged bane of gamers that like to claim they have taste but also the sweetest money pot that publishers and developers have ever encountered in the history of gaming. Just go to the Steam website and do a search for MMOs that are free to play. I'll wait. ... Now that you've done that, take a moment to realize how much they all look the fucking same and recognize that that means two things: That they likely cost pennies to create/maintain and that they are probably turning a profit if they're still active after 6 months. Again, acquiring real data on this is seemingly impossible but if someone is keeping it active, it's making that someone money.
So yes, in a way, MMOs are a dying breed. They've heavily stagnated and the old rules of making money with them turns away both new developers with ideas and the old school of players.
On the nostalgia side of it...
It's an oddball reason and just speculation on my part but this is what I have to offer: The Internet isn't a club anymore. Everyone is on the Internet. It makes it less of a haven for those awkward kids to find other similar people and form groups of common interests. And with how the average age of gamers has increased dramatically, it makes it even more difficult for them. That group of 20-30 people that you "knew" from that game you played as a teenager? That shit is much, much rarer nowadays. MMOs don't really offer the same values and rules that they used to in exchange of being theme parks for cheap money grabs.