The main plot and subplot in King Lear make the play exceedingly complicated, especially since both old men are forced to leave their homes and roam the countryside in different places. Added to this, Cordelia is far away in France and Edgar is hiding out somewhere in the open country. Shakespeare shows a boldness in the construction of this play and a disregard of the Aristotelian unities that is unprecedented. All the characters are scattered in different places. Goneril and Regan are given different domains, so they too are separated much of the time. Meanwhile, Kent is a vagabond because he has been exiled by Lear. Oswald appears in several different places. No one knows what happened to Lear's one hundred knights. The Fool drops out of sight. Shakespeare must have had much confidence in himself and in his audience, especially since he had to present all of this on a small stage with very limited scenery and props.
Shakespeare may have felt he needed a subplot because nothing much happens to Lear after he disowns his two daughters and goes off on his own. Lear can do nothing to regain his power or to get revenge against Goneril and Regan. There would have been, so to speak, "no second act." Cordelia’s invasion from France has a deus ex machina quality about it. Shakespeare devised the subplot to suggest that children turning against their parents was not an isolated phenomenon but to dramatize what he believed was common in human nature. In Lear’s case it was daughters, in Gloucester’s it was a son who thrust the older generation out into the cold.
At that stage in his life Shakespeare was becoming quite cynical about humanity, as he shows in his Timon of Athens, for example. And here is a pertinent quote from his Measure for Measure:
Friend hast thou none,
For thine own bowels, which do call thee sire,
The mere effusion of thy proper loins,
Do curse the gout, serpigo, and the rheum,
For ending thee no sooner.
It should be noted that Shakespeare is not only showing children against fathers but brother against brother and sister against sister, even servant against master. In Act 4, Scene 2, the Duke of Albany, thoroughly disgusted with his wife Goneril, says:
If that the heavens do not their visible spirits
Send quickly down to tame these vile offences,
It will come,
Humanity must perforce prey on itself,
Like monsters of the deep.