I love the Customizer, and I love the decision to require it. For end users, there are no downsides. When I was new to WordPress, I spend hours learning theme-specific settings panels. When I switched themes, that time had been wasted. I honestly thought those panels were a part of WordPress.

The people upset by this decision are the ones who make themes filled with spammy options panels. That is it. There are no other losers in this decision.