Same logic goes for literally any armoured vehicle. Why not instead of "expensive & fancy" modern FICV, get just 7.62-protected APCs that we can even more cheaply buy 10,000 of?
Point is, present that tech allows a modern IFV to protect its passenger from expected autocannons/HMG fire & IED without losing amphibious capabilities... And some composites inserts & 2 kevlar inflates won't add that much cost that you'll have to call it "gold plated".
Well I would imagine we can design an updated BMP with some more protection against 30mm, maybe replace the turret with an unmanned to save weight etc though would be equally vulnerable to plunging fire and ATGMs.
That wouldn't cost much - but a full fledged modern ifv, a Draco, Ascod, Puma equivalent would, as much as a t-90 but not much more protection.
I would argue it's not the sane for other armoured vehicles, especially MBTs. That extra weight and spend on a M1 class vehicle would give a vehicle that can take hits from most Anti tank munitions, far more so than a t-90 class.