I think the main idea of the play, and the reason Shakespeare wrote it, has to do with old age and the way the oldest generation is ruthlessly pushed aside by the succeeding generation. This has been happening with our species for something like seven thousand generations. It is inevitable and necessary, but it is very painful for the generation being pushed aside because they have raised the succeeding generation with love and concern. When Lear and Gloucester are together on the heath they symboiize the older generation. They have lost everything. Their homes are occupied by others, and all their possessions now belong to others who don't even know or care where they are. Both men are looking forward to dying, because there is nothing left for them to do.
I like to read this play as being about identity. When Lear tries to change himself according to an idea, he betrays his true nature. This attempt is doomed to failure because Lear, like all of us, can only succeed in being himself, never as someone else.
The main idea to me is that one has to be constantly vigilant. You cannot get comfortable and let important decisions be made by others. Even as we age, we need to remain in control of our lives. So do crossword puzzles and join exercise classes so you don’t allow your brain to turn to mush and let others make decisions for you when you’re older.
To me, the main idea of this play is that you need to be aware of what is important in your life and you need to value the right things. Lear values the false love that he gets from Regan and Goneril when he really should value the true love that Cordelia feels for him. He chooses the wrong thing and ruins his life.