I think it's because not all content is for all folks. It becomes unreasonable for Square to expect people to pay a sub in order to do a thing they don't enjoy. Ultimately, I think square has kind of spread themselves thin trying to appeal to every possible player, which leads to players who enjoy specific things being kind of underwhelmed by content being drip fed or spread out. There's nothing wrong with folks looking around and saying 'Well, I did all I wanted to do, time to go do something else and drop my sub until they put in more stuff I want.'

Not to say you're trying to suggest people force themselves to do things mind you, but it's an argument I see often enough. Square has a hard job in that regard; how do you keep all the different types of players that you have drawn into the game happy? It's a rough scenario to face off against, particularly when other games specialize things and do them in superior fashions. Guild Wars 2 has excellent Net Code, events, and PVP, but their story is a bit lacking and the RP scene is non-existant. WoW does customization better, but is flooded with negative elements. Other MMO's as they come out will offer more and more modernized, niche content that will take away those niche gamers.

FFXIV is in the awkward 'trying to get them all' placement. Coupled with dated technology and an unwillingness to innovate or change from a tried-and-true method from a decade ago, they are being left behind by more modern approaches. Genshin Impact by the comparison has over 20 mil/month in revenue, and doesn't even have a sub. FFXIV (Factoring for 1 million players at 12$~ average sub) makes 12 mil a month, probably closer to 15~ with mogstation shenanigans and extra retainer fees, etc. They are already falling behind the revenue generation of free games, and that will only get worse.

This isn't to say I hate FFXIV or anything it's just monetarily, the business model is beginning to fail and can only get worse. Content wise, we've never seen content release this slow or drip-fed. The NetCode is still awkward, and the story is questionable for many. This kinda leaves people in a lurch, and as unsubs continue that revenue dive will continue. I want to see FFXIV succeed, I love the game, but it's difficult to justify paying for a game that is just dripping content and phoning in bonus material when its competitors are hungrily providing more modern, and more frequent content.

The revenue angle isn't just a problem for the people who are unsatisfied to crow about; the revenue loss will make it a problem for everyone in the community. This is why I find OP's sentiment so self-indicting. People leaving is good for no one; taunting them isn't sensible to do.

TL-DR; As Yoshi has said, not all content is for all folks. Some of us have already cleared the stuff we're interested in. But I hope others do follow your advice, if there's content left they haven't cleared yet.