Overall, I agree that this would make a better cap system. To be realistic though, this has a 'slim to none' chance to being added into RO2 because of the work required to code in a new system and balance each of the existing maps. Your best bet would be to add this cap system into a custom map, or convince one of the major mods to incorporate it.
As a side note; spawn protection should never encompass an active objective. This is a design philosophy that we try to stick to for all RO2 maps, although there are still cases where it occurs.
Objective A does not turn into spawn protection when A is captured by the Axis (in fact, there is hardly any spawn protection change once A is taken). The only objective on Mamayev that turns from an active objective into spawn protection is the south-western (towards the Axis) quarter of the Central Bunkers. This was done because the objective is very oblong in shape. If I were to push the Axis spawn back any further they would have too far to run to the next objective. In practice, all Russians will undoubtedly have died in that area by the time the Axis are able to take the objective, so it is not much of a problem. In hindsight, I should have split the objective up into two sections to allow for less drastic spawn advances.
Personally, I prefer to have longer run distances from the spawns to the objectives to avoid problems like this, but that does not fit into the design and gameplay style that was chosen for RO2.