I'm exploring two different sources on French history in particular: Barbara Tuchman's A Distant Mirror covering many aspects of life in France from about 1350-1400 and listening to Mike Duncan's Revolutions Podcast specifically covering the French Revolution.
One component that struck me was the seemingly unchanged relationship between the Nobility and the other classes, particularly the well-off 3rd estate. Basically in the 1300s it seemed that the Nobles enjoyed the majority of political power, had reserved political, governmental, and military posts, and were generally dismissive while also fearful of the nouveau riche from the 3rd estate who were slowly overtaking them in wealth, if not in influence. Tuchman clearly indicates, in descriptions of royal politics and the disasters of Crecy and Poitiers, that the nobility's power is receding. In the 1700s, this situations seems basically... the same. Nobles still exert disproportionate influence over government and bureaucratic institutions, still have a number of exemptions and reserved duties (such as tax exemptions and a monopoly on high ranking military posts), and still scorn but also fear the wealthiest and most educated members of the 3rd estate. Obviously in the late 1700s, the nobilities days are numbered, especially in France, but was this a linear decline starting in the 1300s (with the Black Death's empowering of poor workers, or the advent of the longbow, or the rise of merchant classes, or the invention of the nation state?) or something else?
My question is - what changed in the relationship between the nobles and the rest of society between 1300 and 1800? How did the nobility retain such power in the face of shrinking relative wealth, a drop in their martial utility, etc? I know my sources are specific to France, but how did this vary across the European Continent?
I wish I had the time to give this an extremely in depth answer that it deserves, because it's a wonderful question, but I can point you in a right direction at the least. John Lynn's The Wars of Louis XIV, 1664-1714 speaks on this matter explicitly in the opening chapters. You also see this mimicked in Johann Ewald's Treatise on Partisan Warfare where you can see that, even in the 18th century, this transition was noticed.
The short of it is one word: military. To quote Ewald,
"The permanent military establishment proved an expensive but indispensable component of absolutist rule. In domestic policies it served as a means to bind the nobility to the state. . .In foreign policy it formed a readily available instrument for power politics. . .It's ranked and file, however, lacked a feeling of loyalty to the state or dynasty.
Basically it worked like this: In the 1600's, in France in particular, we see the Aristocracy coming to heads with 'the State' in seemingly endless noble revolts in France. To bind the nobility to the state and stop these revolts he integrated the nobility intimately with the military and giving them significant power considering their declining state. Ultimately the nobility was still needed and this was the power boost that pushed them along.
This is just one reason though and a condensed one at that.
One perspective (oversimplified, but still possibly useful) is that the nobility and seigneurage (knighthood) were essentially a military class.
They won their lands and power through being willing to fight for it. In the 13th century (and earlier) they were the essential and predominant military force, when an armoured and mounted knight was the battle winning force, his castle was an essential defense, and it took considerable land and the work of hundreds of peasants to support this human (and animal) war machine.
As military technology changed (longbow, pikemen, musketeers) the mounted knight became less and less essential and powerful as a military force.
The real wonder is that the nobility managed to hang on to power as long as they did once their military supremacy was challenged (and the economic supremacy of land ownership was also challenged).
They did so, at least in part, by becoming the critical officer corps of the non-noble armies which had surpassed the mounted knight. (And, in part, by the continuing value of land ownership, which had originally been an essential part of enabling their fighting prowess, but became an economic advantage).
The third estate, in an over-generalization, was not that keen on fighting and dying. The nobility maintained a martial and warrior ethos. This was one element which helped them maintain power and influence for long after their original battlefield role had become obsolete.