It may be worth adding a little context about 'guarantees' - there are few guarantees with air travel - your seat assignment isn't guaranteed, neither is your meal choice, and ultimately your travel on that aircraft at all isn't guaranteed if you get bumped. For that matter, the old saying goes there are no guarantees in life except death and taxes
The logistics of reserving bassinets in advance sounds expensive to me, and may not be workable due to space on board, and situations where the outbound flight has 20 babies and the inbound none (or vice versa) resulting in expense in carrying extra weight. More work for the cabin crew to do on boarding too which is a very busy time of the flight.
I think sometimes being persistent and sticking at your guns can be an admirable and courageous quality, but it is important to choose your battles.