Space Marines were never meant to be the main force. Maybe the poster boys, but never the main force of Imperium.
Whereas Imperial Guard is the massive but blunt "Hammer of The Emperor", a fighting force that cannot be paralleled in terms of conventional warfare with anything in Galaxy (Tyranids might be numerous, but they lack technology, Orks might be numerous but they lack sophisticated tactics, Chaos is way too unorganized, Eldar are dying race and Tau don't matter on Galactic scale) in terms of sheer, focused strength, the Space Marines are the "Angels of the Emperor", a fast, light but well armed strike force that crashes down at the enemy when it seems everything is lost, and tip the balance of war. Where getting hit by Imperial Guard is like getting hit by a hammer to the face, getting hit by them is like getting a scalpel slammed into your heart by a raging gorilla.
Not even during the Horus Heresy, back during their brightest days when there was more of them in one place than there are now in whole sector, they were meant to be the backbone of the army. They were the tip of the spear, a wall that went through the whole Galaxy and picked off the most dangerous threats while Imperial Army mopped up the less important targets behind them.
Primaris Marines are... they are honestly in a bit of a weird place now. The Marine detachments were usually pretty small, up to patrols of literally one squad flying around a system, and it was incredibly rare (there are exceptions, of course) for a Chapter to focus all their combat might in one place, in which case also they usually enrolled local forces anyway, so they could return to their way of striking the death blow instead of getting tied in attrition warfare like Imperial Guard does.
Meanwhile, Primaris with their heavy stuff... it's... it's like sending M1 Abrams instead of a commando team to assasinate Bin Laden.