I think a lot of people think like you do, but wasn't that true when Facebook did this too? Weren't people upset for the exact very same reason?
And yet Facebook went ahead and did it, it pissed off a lot of people, and everybody got over it. Most users don't even realize that FB is not showing them everything anymore.
I really hope Twitter will do this, I think it would make their product so much more accessible and user friendly, if they manage to communicate it the right way and switch new users into it.
Nobody's stopping them from offering it as an option, like FB did.