I know Artemiz kind of responded, but I will jump in there. A casual is just someone who casually plays. They might be someone who goes to college, works full time, and has a life outside of the game. They play when they can but can't log in a whole bunch of hours. I mean, we sometimes call people at this extreme a "filthy casual" but they are casuals nonetheless. I actually know people like this, some of them in the military. They want to be on but they get shipped out on missions and sometimes in places with no internet connection. So they will go days, weeks, or months without being able to play. Then when they get back and able to play, they go as hardcore as their schedule allow them.
Then there are people who have obligations but nothing as significant as previously mentioned. They take care of real life obligations and then hit the game when they can in their spare time. It doesn't equate to as much as they may like, but they get on for their 1-2 hours a day, maybe more on the weekends or days off.
Lastly, there are people like me who are disabled and have absolutely no life. Consider us NEET or whatever, but we spend 8-16 hours a day playing FFXIV.
Now, how would you suggest you'd release content that would make the game enjoyable to all of these people? That's the challenge. I think FFXIV has done a very good job in that, by trying to do synced dungeons, roulettes, etc. They try to keep more active and higher leveled players interacting with others so we can all progress. But they also add in things like Extreme content and all to keep the people with more time busy as well. I'm not sure what you're calling "dumbed down" but I do know they try to keep things balanced. Hardest tier stays hard for everyone but once it's no longer most difficult content or they are about to release next phase of stuff, they sometimes make it simpler so newer people can get geared and ready. They don't want people stuck trying to gear up for months just to get into new content. They want to make sure we ALL can find groups and advance, not get stuck lagging behind others with nobody to help.