It's been happening for years already. Out of the original people on this site from 5 or so years back, there are maybe 3 or 4 left. And these are not just people that have left the public hobby, most have left the hobby altogether.
Majority of people who keep tarantulas, start off with a big bang and within a year and a half get bored and leave.
I know it's difficult to compare to America, but there new people tend to approach the hobby at a slower pace. First research, buy a few starter species, grow in confidence, buy something more challenging and after a year or so start moving on to the expensive stuff. Here newbies come on and buy a balfouri within 6 months, find out Metallica is the most expensive blue spider, either buy it and get bored within 6 months or decide its a met or nothing and leave the hobby when they can't afford it.
Obviously these are generalisations, but the stats would tell you that generally they are true.
Other than that, you obviously get the people who's lives get in the way. New partners, kids, bills, moving house, just need cash, etc. if I had to sell my whole collection to pay a doctors bill, I would too.
Then there are the people that have left the public hobby because of politics. That is happening across the world, not just here.
Sad to say, and don't flame me, but I would be impressed if 1 of the 3 people that posted before me on this thread are in the hobby in 2 years time. That's just the way it goes. Tarantulas tend to be a stage that passes for most people.
What is more worrying to me is the fact that new people tend to go for the higher end spiders as a start, instead of starting small. Speeds up the natural cycle of users.
Obviously all of this is my own opinion...don't hate the player, hate the game.