I can see problems for corporate users. Because corporate users have multiple licences, they can have the same user having access to many versions at the same time.
I already have a user that has unloaded V15 because it deleted his V14 copy. I have no problem with the user needing to use two licences at the same time (from my precious pot of floating licences) - I do monitor and manage usage, and I would expect, even require, that users, take a stage approach to upgrades. We still have users that depend on mathcad sheets written in V8, so we do need to check for consistency on a side by side basis.
It will be necessary for there to be a separate install process for corporate users that does not mandate the removal of V14.
Note: The FlexLM licence file for MathCAD V15, requires that the version number listed in the FlexLM licence file is 14.1 or greater. The licence file's version number was 14.0 for MathCAD V14.