You can make sub-optimal combinations, but D&D is a team game. If you build a Barbarian that can't deal damage, or a Wizard who's spells never land, you're letting the rest of your team down.
> The rules should not prohibit from making such selections.
The new rules give you _more_ freedom to choose a suboptimal build. You can even play a Gnome with low intelligence under the new rules, something that was impossible before.