Oil change intervals should be strictly based on driving style.
For me I drive a very short distance to work on a daily basis. The car doesn't even get a chance to warm up. While I try to make sure I go on a cruise after work to get things warmed up and flowing that's just not enough. When I get to work and shut the car off cold there is a lot of unburnt residual fuel that gets to sit in there and leak into the oil. Gasoline breaks down oil badly, even synthetic oil. On an extreme driving routine like this 3-4k miles would not be a bad plan.
Now if you are one that does a 30 mile highway commute every day, 5k is overkill. You could easily push it 7500-10k miles. The car gets up to operating temperature and stays there for a prolonged period, it's able to properly burn all the gas it is using and prevent deposits from sitting in the cylinders.
For your average driver I say 5k is a reasonable number.
In the most extreme case of highway driving 15k miles wouldn't be wrong but this is like truck driver style driving and most passenger cars are never treated like this.
Also just because a shop finds problems with a car when they are doing your oil change doesn't mean they are lieing to you... cars break deal with it. Most shops make little to no profit off of the oil change and if they can get you back in the door for something wrong with the car then they will. This gets twisted by many consumers to be a rip off but hey if the car is broken... well... don't fix it if you don't believe it but its your car and in some very rare cases your life that could be at stake. I know I don't want to have a control arm / ball joint break driving down the highway at 60-70mph.