Not sure what you mean by "dropped"? Mach4 is the replacement for Mach3. Mach3 became too complex to develop further, so Mach4 needed to be written from scratch.and more than a little worried that it might get dropped like Mach3 was since its not being widely adopted.
OEM customers are keeping Artsoft in business, from what I understand. My guess is that they still sell more Mach3 licenses than Mach4 licenses.
As for the Acorn, do your research, and make sure it'll do what you need. I know a few people using them, and have heard that it still needs some work to be a good fit for router users, but they are working on it.
I'm building a router using DMM servos, and am running UCCNC with a UC300ETH and UB1 breakout board. I've had a Mach4 license for 3 years, but have no plans on using it.