Well, if there's no competition, there's no need to continue making good/original/interesting products. If Disney owns everything, and that's all people have to watch, Disney would no longer feel pressured to keep making top-notch products. Not to mention that different companies have different flavors. Say what you want about Dream Works, but they have had some wonderful movies that Disney probably never would have made and never will make.
I like certain Disney movies, but they tend to be incredibly safe and sometimes lacking in... Like, visionary creative flare. Sometimes they really do feel like a product made to please everyone rather than a piece of art made to please its creator, you know? Disney knows what sells (Marvel humor, happy endings, predictable characters/character arcs) and they don't often stray too far from that. I think that's why they've made so many Disney Princess movies that are really pretty similar to each other.
You won't ever get a Princess Mononoke or a Prince of Egypt out of Disney, and I think that's a shame.