There are facts about human behavior which can only be learned by actually testing them. People are complex in ways that you, whilst sitting at your keyboard, cannot figure out.
After you have the engine running, the marginal cost of an A/B is probably smaller than a focus group test. So if you really believe in focus groups for the results and you are also ambitious enough - probably this means go for A/B.
It’s very common to think something is a good idea based on focus groups and internal feedback and then learn it’s a bad idea from launching it (or running an AB test).