Furthermore, in my experience the data doesn't necessarily mean what it's believed to mean, with the favored explanation for the data being assumed to be the correct explanation. (e.g. "We want more engagement. This data shows users are staying on this page longer, that's a measure of engagement." It may instead be a measure of your users getting confused, lost, etc.) My point here is that an earnest attempt to be data driven doesn't eliminate the matter of bias. Being cognizant of possible bias when evaluating the data is important, otherwise you'll have a false sense of confidence.
What you can get feedback for is how eye popping or outrageous (no F keys!) your suggestions are. People in the office will react, and they are the ones who will decide what actually gets shipped.
On another topic, putting someone's job into quotes just isn't a good look for any argument. If you have to straw man to get your point across, it might not be that great.
Data doesn't need to be "track everything the user does and do A/B testing", it can come from many sources: user behavior studies done in the past, cognitive psychology, experience gained from mentors, etc.
Would you make the same "they do it every day so they must be right" argument for engineers? Product managers? Stock brokers before data-driven trading? Doctors before the rise of modern medicine?
Those designer might actually be very good one's but they might have different goals. Their design might be all about eye pleasing hoping that mos potential customers will glance at it, like it and buy it. Ergonomics might not be their concern at all.
There are other UX designers though, making GUI for pilots, nuclear station operators etc. etc. They have somewhat different goals. I feel that my brain digests what they do way better then the stuff coming from a first group
I come from a different background where I learned to value reaching the goal in most clear and shortest way
On the other hand, since (insert obligatory joke about $1000 monitor stands), losing customers is no longer their worry.
Actually, would this be so bad?
In my frank opinion, many designers hold a position of subjective judgement and thrive on the power that comes with that. This is like a developer making arbitrary decisions about a product because they like the technical implementation better.
Data, and actual user preference, should always be the key decider. Not some arbitrary rule-decider.