because they didn't want to switch careers part-time into something far outside their original job description. that sounds borderline illegal as constructive dismissal anyway (ie that job you took? we're cutting the hours on that to half, good luck seeing as much career growth opportunity as you had expected when signing the contract)

Because if they were anything like me, and the boss unilaterally decided I'm going down a different career path, they would have eventually quit.

I think the implication here is that he otherwise liked the position. Sure, you can always hike and try something new, but if you like what you were doing before, communicating helps. Don't assume that a change is immediately Unwaivering Word Of God.