To play devil's advocate, without A/B testing a lot of decisions would be made with insufficient relevant data, and lead to subpar results that affect the many negatively form the road.

It is necessary to have a control group, just as in trials for new drugs.

Depends entirely on the stakes and whether personal data is involved

Agreed and I can't wait until they regulate this stuff out of existence. It's absolutely hostile software technique that is deeply anti-human.