Zugg, all credit due, was too in love with his favourite programming language to move forward and get with the latest platform tools. Delphi and maybe Pascal too, I think it was? Third-party libraries out of date, but too embedded fixes and hacks for him to update to the latest releases, etc.
zMUD has many, many features that less than 5% of the userbase take advantage of - but you can't hassle a guy for enjoying his hobby and letting his creative ideas run unrestrained. Yes, I bought zMUD and urged others who enjoyed using it for years to do the same - especially since the proviso was that you'd get updates for free, forever.
CMUD is an upgrade to zMUD in that it's another MUD client written by the same guy. But because it's a rewrite from the ground up and for an (apparently new) platform, he's made the decision to release it as a new product and therefore not a free download for registered zMUD owners.
I'm happy with what works and I have no guiilt in continuing to use the zMUD client I've paid for. I wish zugg all the best in selling his new client to the next generation of Mudders.