Don't be too trusting of dealerships, I've worked in service departments my whole life, it is usually a total crapshoot. Whether they actually flush the system or just swap the fluid in the master cylinder depends on how dirty the fluid is, who is working on the car, what kind of day they are having, if the service department is busy, if you are having other work done, etc. I wouldn't blindly trust that they will do an actual flush. That said, all I do with my own cars is drain and refill the master cylinder once a year. The master usually makes up 40-60% of the system's capacity anyways. Changing out just the master cylinder is certainly better than doing nothing at all. And honestly if you do that, you are probably doing more than 90% of people out there. You can do it yourself even if you aren't the least bit mechanically inclined, and it takes all of 5 minutes.
It is true that the worst fluid is near the calipers themselves, but it can be bled out easily. Crack the bleeders and just let the fluid closest to the calipers drain out until it starts appearing a little cleaner. I won't explain the whole process, there are a million YouTube videos on it (EricTheCarGuy has a pretty good one, on a Subaru ironically). My point is, you don't need to flush the entire system, just get the worst of it out.
Also, the second worst area for fluid contamination is in the master cylinder, so again, if you change only the master cylinder fluid, it's a lot better than doing nothing.