No, USF need mortar vs buildings, especially vs HMGs, otherwise you are forced to get grenades while OST/SU have flamethrower, as for OKW, they can get incendiary nades with teching so they don't sacrafice teching for nades like USF.
I think you and Firesparks are kind of saying the same thing; not having mortars forces USF to get grenades.
However, where your opinions clash is that Firesparks (and I) believe that USF needs to be forced to get grenades, otherwise the ensuing Cpt & Stuart rush will cause USF to almost-always faceroll OST.
Basically, a good compromise solution would be to place the mortar behind nade tech. If the enemy knows how to camp properly, grenades and smoke won't help you (and you will also run out of munitions). The mortar is going to be a tool that allows you to deal with those situations more cost-efficiently.
Another compromise approach is moving the mortar behind Lieutenant tech, which has been dead for so long. A side-effect from having mortar and pak howie disjoint from one another will give some people (Lieutenant-skippers) a reason to field the Pak-howitzer.
Yes, Rifleman-spam opening sucks so hard. However, it's either that, or nerfing the Riflemen (and then, how is USF supposed to survive with their paper tanks?).
Finally, regardless of the locking option, this begs the question: How come Soviets get to end up with the crappiest mortar to begin with?