It is a common misconception that the "expert" knows the best. Expert can be a trainee, or may be motivated to make more for its organisation or have yet to encounter your problem.
On the other hand, if you are using your car for a decade and feel it needs a new belt - then get a new belt. Worst case scenario- you will lose some money but learn a bit more about an item you use everyday.
I am a qualified mechanic. I no longer work in the field but I did for many years. Typically, when people 'trust their instincts as a user' they are fantastically wrong. Off by a mile. They have little to no idea how a car works besides youtube videos and forum posts which are full of inaccuracies or outright nonsense and they don't want to pay for diagnosis.
So when they would come in asking for a specific part to be replaced with no context I used to tell them that we wouldn't do that until we did a diagnosis. This is because if we did do as they asked and, like in most cases, it turned out that they were wrong they would then become indignant and ask why we didn't do diagnosis for free to tell them that they were wrong.
Diagnosis takes time and, therefore, costs money. If the user was capable of it then they would also be capable enough to carry out the repair. If they're capable of carrying out the diagnosis and the repair then they wouldn't be coming to me. This has proved to be true over many years for everyone from kids with their first car to accountants and even electrical engineers working on complex systems for large corporations as their occupation. That last one is particularly surprising considering that an engineer should know the bounds of their knowledge and understand how maintenance, diagnosis and repair work on a conceptual level.
Don't trust your instincts in areas where you have no understanding. Either learn and gain the understanding or accept that paying an expert is part of owning something that requires maintenance and repair.
If you don't trust the expert then why are you asking them to fix your stuff? It's a weird idea that you'd want an idiot to do what you say because you know better.
If they're asking the mechanic to do X and they understand the mechanic is just doing X and NOT venturing to fix your problem. I guess that is fine.
I agree though it sets up a weird dynamic where folks might come back to the expert and complain a problem isn't fixed, but that's not what they asked for / they broke the typical expert and customer dynamic.
In my experience the best thing is to then convince the expert you are right, using your own expertise.
If your mechanic is too stupid to recognize the problem after you explain it then you don't have a mechanic, the set of hands you are directing is basically unskilled labor.
On the other hand, if you are using your car for a decade and feel it needs a new belt - then get a new belt. Worst case scenario- you will lose some money but learn a bit more about an item you use everyday.
Experts don't have your instincts as a user.