Game told you about the lock in when they introduced it. Was shown as a 1 2 3 step program.
Supposedly the game is supposed to try to keep you at least at 50% of your primary roll. It may be flawed, but you shouldn't be going 10/10.
People who got their main roll in the old champ select often had faster computers. They could call it sooner. While you might get your roll. Someone got forced into a roll they're not as familiar with. Yes, it sucks you might get stuck with that other roll. But at least you don't have to worry 'as much' about your team sucking at their roll.
New champ select creates a lot less pre-game stress. Something that was very much needed. A difference big enough that people were begging for it to come back when they took it down for a short time.
Making an option to only get a single roll at a higher queue time is an option. But chances are too many people would do this and hurt the up sides to the system.