when you buy 15, you are given prime. Prime is marketed as mathcad. Yet, to work in prime is much different, and since I am comfortable with 15, that's what I (and many others) will use. If PTC is going to abandon 15 users -who make up 90% of the user base, maybe they should say so.
and as far as not being able to open a newer file with an older program, autocad is not a good example as it is very buggy. Maybe providing a way to convert prime to 15, would make good business sense. If not, maybe prime should not market itself as mathcad. Robert Jackson is correct, PTC appears only interested in a money grab on the backs of old mathcad users.