Amen, and it's also about making way for newer technologies and fashions. Early BRDM with ATGM were really crappy in hindsight, but ATGMs were both: well received after the Yom Kippur War and clearly to be improved upon in the future. Since reactive armor wasn't yet available, HEAT warheads, which didn't rely upon velocity for kinetic penetration offered a distinct advantage of scalability and weight. It's far easier to mount a couple of ATGMs on basically any chassis and later mount bigger ones than put a new, larger caliber gun onto a dedicated chassis. And while the German Bundeswehr ultimately rearmed many of its Kanonenjagdpanzer with TOW to become the Jaguar 2 tank-destroyer, the Soviets preferred to use BRDMs and ultimately the BMD family for its VDV troops.