I fully agree with you. Even if the autoreplace solution makes the game much easier, I have exactly the same problem when a new vehicule is available (I have 35 bus lines and 20 truck lines).
Another option would be useful : autoreplace to the new type immediately after changing the vehicle type.
At the moment, I change the vehicle type in the menu, then if I want to switch immediately to the new type, I have to select the interval to 25%, wait for all vehicles of the line to be changed, put the interval back to 100%.
Once this is done, you still have to check that the number of vehicles is still in line with requirements. This is especially true for truck lines where you need less and less trucks with each generation of vehicle to get the same service.