We've considered creating a web-based solution for GraphixCALC too, but not as a replacement. The plan, if acted upon, would be to offer both a site-specific option as well as a web-based. This way you can choose which works better for you. Some like a monthly subscription and some like a one-time cost. It is still under consideration, but from what I've seen, most people want a one-time fee, though much of the industry seems to be moving towards web-based monthly subscriptions.
FYI, GraphixCALC customers receive lifetime updates for that major version they are on (i.e. 2.x users get all 2.x updates, regardless of the time-frame). So far, we have only produced 1 major version update since Pro 1.0 was released 12/2005. Point is, major versions take a while so you won't be dinged every little while for an "upgrade" fee. Also, we offer a 25% discount for current users to go to the next major version (i.e. 1.x users get a 25% discount for 2.x). There are no other fees other than a per license fee beyond the initial ones purchased, which are only $50 each.