[nine_k]

The nobility used to be the dedicated warrior class, the knights. This secured their position in the society and allowed them to rule, by coercion when needed.

Once they ceased to exercise their military might, some time around 17th-18th century, and chose to live off the rent on their estates, their power became more and more nominal. It either slipped (or was yanked) from their hands, or they turned capitalists themselves.

[achierius]

The problem is your timeline: in actuality, the nobility of Western Europe lost their independent armies by the 15th century, solidly by the 16th, and thereafter held on to a military role only through participation (as officers) in the state-run standing armies that developed thereafter. Yet for centuries they held on to power: in France, until the revolution, and in Britain, until well into the 19th century. Great read on the topic: https://projects.panickssery.com/docs/allen-2009-a_theory_of...

"Living off of the rent of their estates" was enough to remain in control of the state for centuries. Only the birth of capitalism and thereafter the industrial revolution allowed for other actors -- the bourgeoisie -- to overtake the aristocrats economically.