Less power for republics? I'd think the inability to benefit from legitimacy and the inability to have PUs/inheritences is a pretty damn fair tradeoff for more control over rulers.
Legitimacy and royal marriages carry their own drawbacks (low legitimacy and succession wars, respectively). Not having access to a monarchy-specific game feature and a minor diplomatic option isn't exactly a drawback.
Inheritance is chancy and more often than not you're going to find yourself in a succession war rather than inheriting France.You're underestimating diplovassilizing and RM. I managed to own the entirety of France and Spain as Burgundy while controlling the position of emperor in the HRE without declaring a single war, or gaining infamy.
Sure, from a total conquest standpoint, there is a definite advantage for a republic. If you don't want to go on a massive conquest spree, monarchies are typically better, as they allow you to expand peacefully, gain vassals and provide a boost to relations through RM.