You're confusing not perfectly balanced for not balanced. Right now, if I want a dragonborn breath weapon, I get the rest of the not good kit with it. But a hybrid system lets me trade that for better things. Why would I EVER pick a regular dragonborn when there is a hybrid that will be better (even situationally, like water breathing in a high seas game)? There is no cost to being a hybrid, even multi-classing forces you to slow down feature progression as a cost.
Once a hybrid system enters play, you will never see full blooded species on PCs again. The allure of choice and customization will mean everyone takes it. Regular humans, elves and such will be the minority without specifically tailored species traits to keep it in check.