It's confirmation bias.

Adventurer in Need is an extremely bad metric and dissatisfaction with healer changes is only one variable.

There are two very important and much more impactful variables in play:

1) New DPS/Tank jobs, which many are checking out. At release, people were going as far as logging out next to the unlock NPCs to play the new jobs.

2) It's leveling season. Everyone is leveling jobs. With 10 dps jobs, 4 tank jobs and 3 healer jobs, which do you expect to see most? Are you queuing for Leveling/Alliance/MSQ on your level 80 healer main or that other job you're trying to get to 80?

Because I want free materia, I'm waiting for Adventurer in Need to level up AST. I've seen Tank in Need and DPS in Need plenty of times in Leveling/Alliance roulettes. But you won't see me going around claiming that no one plays Tank/DPS.

When I'm queuing for leveling roulette on my RDM, is it because I hate the healer changes and I'm quitting healer? No, it's because I want to level RDM.

If you want to make the healer shortage argument, you'll have to account for as many variables as you can identify if you want to be taken seriously.