During the high middle ages knights were not just relevant, they were considered the dominant force on the battlefield. This changed after the Battle of the Golden Spurs (1302) and the Battle of Morgarten (1315) and a few later similar battles.

In all of these infantry was triumphant over cavalry, with the use of pikes in formation. This meant that large numbers of cheaply equipped and shortly trained soldiers could easily defeat smaller numbers of well equipped men on horseback with long years of training. Note that in the battles mentioned above the winners did not have a significant numeral superiority, though they did use the terrain to maximum effect. But the more general point is that a small elite force was no longer a grave danger to a larger less elite army.

This did not make cavalry irrelevant, it retained a various roles on the battlefield that require great speed: scouting, rounding up fleeing enemies, flanking enemy positions, etc.

As to your second question, I'm not sure there is a sensible comparison. A man became a knight for the most part because of his circumstances, and they were radically different to those of anyone today.