Except it wasn’t the serfs who became the bourgeoisie, it was the burghers (aka the merchants and bankers) that became the “first” capitalists. They over time supplanted the aristocracy as the ruling class, through a variety of methods, many of them violent.

And the nobles didn’t give away their land. At the time, the majority of land was held in common by everyone, with temporary ownership being based on who worked and maintained it. Then with the transition to capitalism, the burghers (now becoming the bourgeoisie) with the aristocrats put all of the commons up for sale, leading to most of the land being bought by the bourgeoisie and most of the rest being bought by the aristocrats, with a tiny percentage going to well off farmers.

Mercantilism is also still under what we consider feudalism, it was merely the transitionary phase out of it, as the social counciousness changed starting in the 1500’s. And culminating in wave of revolutions in the 1600’s to the 1800’s, bringing the material conditions to par with the social consciousness.