Tank destroyers were not part of an initial army order-of-battle i.e. they were not a design of choice. They came about because;
1 – it was necessary to supplement the MBT’s cheaply as losses would have been unsustainable otherwise and the number of gun tubes in the field too low.
2 – it was a way to mate a powerful gun with a small or out of date or captured chassis.
3 – Germany was suffering from availability of resources, so the Jagd / assault vehicle eased the pressure on resources and cost.
4 – German Assault guns were initially intended as mobile supporting artillery platforms and belonged to the artillery arm – circumstances caused them to become re-purposed into more of the tank / anti-tank role.
we are in that position again, modern armies have full Orders-of-Battle for their peace time roles that include MBT, missile platforms and auto-cannon, but at £4 Million pounds a tank, after the first weeks / months of war, who knows what contraptions will be thought up to plug the gaps of diminished orders-of-battle.