It is definitely possible, and even has a clear financial and reputational incentive. But a lawyer only needs one expert witness and can pay that expert. And

It is definitely possible, and even has a clear financial and reputational incentive. But a lawyer only needs one expert witness and can pay that expert. And then that expert can use his testimony as validation of his skill or a new concept. So the expert witness would also have financial and reputational incentives to defect from the licensing board's dogma. Many people have heard of doctors and psychologists using roles as expert witness to push their theories. That leaves the risk of a board ousting (or implicitly promising to oust) people based their expert testimony in court. Which would fall back onto the board's rules about revoking licenses. Lawmakers would likely get involved if any funny business happened; revoking isn't an obscure bylaw, but one of the two major purposes of the board (giving licenses being the other). I'll admit this hypothetical gets too complicated and delves too deeply into politics for me to guess likely. Do you know of any historic examples besides police unions? I'll admit they match your worry, but the police benefit from massive public support that borders on mythology. So it's hard for politicians to do much about those problems.