The decision of making Harmattan available on the N900 can be either come from a business or engineering point of view.

From a business point of view, it could happen that a decision comes from higher up saying "Make Maemo 6 available on the N900 no matter what", in which case engineers will make it happen.

From an engineering point of view, Maemo 6 could be developed until it reaches a point where it can be seen if it is feasible to make it available on the N900... and let business guys pick the broken dishes if not (angry customers, trade-in option, etc).