Good points. 75s might be ineffective against heavily entrenched targets like in the last war but in this war warfare is way more mobile (=targets in the open) and they are just barely light enough to be man handled while capable of taking out even heavy tanks(for the time).
15 cm heavy howitzers had their origin (at least in Germany) in tests against field fortifications in Germany (Meppen, I believe). It was concluded that the notional field fortifications would require a 15 cm field howitzer to defeat. (Kinda like the CRISAT target and PDWs.) 15 cm howitzers got much higher muzzle velocities, much more streamlined shells and thus much more capable of penetrating soil and logs by 1916 already - but the field fortifications were simply adapted to common opfor artillery pieces in both WWI and WWII.
There are guidances from WWII about how to achieve a specific end (suppression, destruction in a specified area etc.) with both light and heavy field howitzers, showing that the two were indeed considered substitutes in both ways. A look at the figures showed me that light and heavy were very close to each other in the first minute of firing (thought he costs of the mechanical fuzes complicates the costs issue). The heavy howitzers had advantages regarding sustained fires (same rate of 1 or 2 rpm for 15 cm and 10.5 cm). 10.5 cm howitzers on the other hand were capable of defending themselves against tanks again with HEAT shells' arrival, though.
Smaller calibres are more efficient for fragmentation (hence the ICM approach and frag bombs of WWII being 10...110 kg bombs only while GP bombs weighed 227...500 kg).
15 cm HE is also dangerous to tanks (particularly thin-skinned AFVs, but also MBTs). Yet by the 2000's preformed fragmentation shells had become the new normal, and those fragmentation patterns are about maximised effect against soft targets. Modern 155 mm HE is no more that dangerous against AFVs with indirect hits because it produces no large fragments.
In the end, the entire heavy howitzer thing was very path dependent. It was nowhere near an optimal calibre except maybe during the Cold War when we used ICM.