Very likely the interviewer was posing to you the exact design that they have already implemented, which they did for good reasons, and very likely your trivial solution would not scale up to the complexity of their needs. So you might have proved you could simplify the problem at hand but failed to demonstrate that you would know how to deal with more complex requirements of a larger system. For example if the commands themselves are quite complicated and have their own relationships and state then how would that be modeled? Eg. how do commands with shared behavior share it? Of course it can all be done in a functional style if you are determined enough, but it could be somewhat awkward and sounds like it wouldn't fit at all with their style of programming. I assume you probably didn't want the job in the end but if you did I would say you didn't do yourself any favors.