OS, first of all, you don't know what the financial/family situation is with any of those actors unless you know them personally. The ones you mention left the show AFTER they'd auditioned and been offered other roles. Leaving a well-paying job isn't easy for ANYone if they don't have another one to go to.
Cary didn't leave because he got Taboo. He had already been cast in Taboo when he came back that summer for a short fill-in run as Roger. Colin (hasn't actually been in Rent 'forever', just 2 1/2 years, which by Rent standards isn't all that long

) left after he'd gotten his new show. And even HE has left and returned after doing smaller shows!
The issue of 'recycling' is not the same issue as the one involving Matt. He hasn't actually been in that position. When he took a bit of time off when Joey and Drew were there and then returned, it wasn't like he'd been gone from the show because he decided to leave for a period of time. He also was 'around' for periods of both of their runs.
Recycling has happened with Rent in its entire almost ten year run. It's nothing new and I'm not sure why people get so upset about it. The fact is that sometimes when they've brought new people in, they have not been able to fulfill the requirements of the role. It's happened more than once and their contracts were either not renewed or were bought out. I can't see any problem with bringing in an experienced actor to play a role when you know they're going to be able to do a good job. The number of returning castmembers has ALWAYS been higher than the number of new people cast in the show, in any given year. If you think it's bad now, you should have been around when the Canadian, Angel, and Benny tours ended. :)
I especially don't get your example of Will Chase, and complaining about that decision. Will hasn't been in Rent for YEARS, was a swing when he was with the show, and I'd bet that 99% of the fans now, never saw him before. His casting for the month of January was brilliant, in my opinion. He's the best thing to hit the Neder stage as Roger in a long, long time.