2-D is also a way of earning money, so I don't see the difference, personally.
Ever since companies switched to 3D, they've put less and less effort into the plot and characters and making tons of sequels.
I highly doubt 3D had anything to do with that. There's a lot of 2D stuff in which not as much thought was put into them, or the characters. The advent of 3D didn't change anything; it just continued an already existing trend. There are still quality 3D movies as there were quality 2D movies. But just like how there were awful 2D movies, there's going to be awful 3D movies.
Sequels existed before the advent of 3D movies as well. 3D movies didn't cause this trend; it already existed before it, way back when books were the only medium (like hundreds of years ago). 2D movies also had their fair share of sequels.
My main point isn't to say "out with 2D, in with 3D", as I prefer 2D. However, I feel it is unfair to say that 3D is of lower quality, as I've seen a lot of 3D work and they can be really good. You can say that 2D had more good movies, but the main reason for that is because 2D was around longer. For every 2D masterpiece, there are a lot of less than appealing 2D movies. 3D is going to be the same way.
As I stated, I want both to stay, 2D and 3D. I enjoy both styles (albeit 2D more) and feel they both have something to offer.