Nothing, because there wax no downfall. The feudal system evolved, like all societal systems do. Subsistence became less of a struggle, surviving population increased, improving technological and manufacturing methods made for more opportunity for the working man, who was no longer so tied by necessity to the soil. And the English aristocracy, as well as the 'squirarchy.' began to see that people worked better, and everyone benefitted, when they were allowed to profit from their labors. Living closer to the land than did the French aristocracy, for instance, the English were quick to see the benefits of freedom, for themselves and the 'common man.'
There are a horde of reasons and causes, but chief among them were the lessons the Barons' learned from their own struggle, with the erratic and conniving John 'Lackland' Plantagenet, to win some independence from his whims, and a clear delineation of the prerogatives and responsibilities of both Crown and Noble. Also huge in the evolution were the technological advancements, with their attendant industrial development, and the increasing level of education among the 'lower classes.' Though, actually, education wasn't all that common till after the Renaissance, even among the upper classes. The more educated a person, the harder it is to 'keep them down on the farm,' and the higher they may look to progress, in a free country, on their own merits and efforts.
This would be the primary reasons for the morphing of the feudal system in England. That and the fact that, as the need for actual combat-ready leaders declined, so too did the rationale for most of a population laboring to support others for the sole purpose of being trained, ready and available to fight at a moment's notice.
You can see that it's a whole lot more complicated than just a downfall. Its' all about inter-dependencies, about surviving and how best to go about it. As times changed, so did the feudal system, right along with the people who comprised it, both the top and the bottom of the spectrum.