It's also a bit of a red herring IMO. As a general rule people buy into an ecosystem and then stick with it for many years. DCC systems aren't, in the main, modular where people are constantly buying bits that go with your control system. If you buy a ZTC today and they cease trading (again) tomorrow you don't really lose much. Worst case is that it fails within the warranty period and you'd have to pay someone to fix it or in the event of a total failure of the motherboard you may have to search NOS or second hand items, but that's a lot of ifs and "what-abouterry".
As I've said I'm happy with my 511, I've started sniffing around other things because it's starting to play up a bit, but Taunton Controls do a very reasonable repair service I'll probably take them up on at some point. It has some shortcomings (and I don't use sound, so I'm probably not constrained by some), but it's a good controller. To loop back to (and arguably contradict) my initial point I have a 622 handheld controller for it, so I'm heavily invested in the ecosystem and slightly reluctant to start again with something I don't actually like in use.
FWIW I think the Digitrax Zephyr feels really cheap in use, not a fan at all.
As others have said, if you like the ZTC, and can afford it, then go nuts! The issue at the moment seems to be actually procuring one!