A/B/n testing let's you explore a search space. There are issues that can come up from that, but IMO it's a better option than this Dilbert comic[0].
The testing advocates I've worked with have an attitude of "I don't know what will work - that's why we're testing". I have not seem much of that attitude from most other individuals/teams involved in decision making -- they have preferred to say "this will work", and get angry when I say, "compared to what? how do you know?"
edit - fwiw, I totally agree with understanding the limitations of testing. You have to know what it can be good for. My argument is that it's actually better for long term strategy than most other decision making processes I have experienced, which usually boils down to "gut based".
Unfortunately, it's not really possible to prove/disprove an opinion on what a design element will do.
Additionally, optimization testing allows you to try out different options. Going with someone's (educated) gut feel is a worse decision than exploring the option space.
Ideally, you'd test everything. In the real world, resource constraints apply, so at issue is how to establish an internal discipline where facts trump opinions, and opinions can be tested then either discarded or confirmed.
When it comes to design, there often aren't facts (one may even suggest facts don't exist when it comes to design). Study the history of art to contest that. Otherwise, listen to users like they are humans, and see whether it matches with the designers advice. Simple.