So, what about the path between "complete rewrite" and "invent a proprietary intermediate language": doing what they did at the end (running a permanent transpilation and committing the result) at the beginning? They could have just moved from straight ASP to straight PHP, and then later done it again to move to straight C#. No rewrite necessary, no technical debt necessary.
Path dependency. At the time, PHP was not as easily deployed on Windows as ASP, and so the original design was set up to give them PHP on *ix and ASP on Windows.
