I have to disagree somewhat with you comment about adapting an obsolete tank. The figure about the Grille and Panzerjäger 38(t) are misleading in that respect. The early Grille and Panzerjäger 38(t) were just conversions on the original chassis. Later versions with the setback guns and redesigned superstructure were also redesigned automotively. The engine, originally in the rear, was moved to the front to allow lower and more accessible fighting compartment. Also during production the engine were redesigned and upgraded.
As for the Hetzer, it was a totally new vehicle using only components of the 38(t) series.
It was not using an obsolete tank on a poor man's budget. It was, as you indeed said, a good tank in 1939-1940. But once it was outclassed as a battle tank the German reused the chassis and basic design for a multitude of variants and one new design because, fundamentally, it was a technically well designed vehicle on par with or exceeding the equivalent German and Allied vehicles. Look at the numbers, more than twice as many chassis were produced for varients as for the MBT, assuming some were reworked MBT chassis and not counting the new Hetzer design.