It's called nostalgia. The problem with anime fans in general is burnout.

The shows when you first started watching are "the greatest," you have more tolerance for flaws, and you enjoyed it all because it was fresh. The problem with a lot of anime fans is over time, they lose interest. The later stuff becomes "derivative," "shitty," "ripoff," "poor-man's [fill in blank]," and so on.

Older anime fans are the most jaded fanbase in existence. There's some real modern gems, but you'd never know it talking to anime fans in their 30s or 40s, or those who have made a job out of it. They seem to only enjoy the meta-fanbase shows out there these days.

Specifically, regarding shows like HxH and FMA, you have fans who watched the older version of a series that was rebooted and improved, but they are either infested with nostalgia, or burned out to the point that they won't even try it.

Don't get too wound up about it. It's just older or former anime fans being how they get.