You are asssuming that because a vehicle can carry fewer dismounts a mech-unit will have fewer infantry with IFV rather than APC. This is not the case. The lesser ability of IFVs to carry personnel means that you need more vehicles to move a given number of infantry. Which makes your mech-inf unit more expensive. (where you used to need 3 M113 APCs to move a platoon of infantry around today you need 5 M2/3 Bradley)
That can be true, but is not always the case. The Soviets and the Indians went with 3 viehicle platoons with IFV's.
Indian Armor TOEs
However, you are right that upfront cost goes up with IFV's. In a lot of situations, the increased cost and reduced infantry is worth it. I was simply saying an IFV equipped unit is not automatically superior to an APC unit in all settings.
On a side note, I think 5 vehicle platoons are a mistake, as are 3. 4 is the obvious right number. it can easily divide and leave each unit with a wingman. It can do leaping or bounding over watch by 2's. It can cover the cardinal points etc.
You might have some infantry becoming IFV drivers , gunners and commanders if you make the switch from APC to IFV and a limited by a requirement that the total number of unit personnel does not grow. (from 33 infantry + 6 crew = 42 personnel to e.g. 24 infantry + 12 crew = 36 personnel rather than 30 infantry + 15 crew = 45 personnel).
Uhm 33+6= 39 last time I checked.