The laws of physics haven't changed

Your salesman is right.

The laws of physics haven't changed. The smaller (and obviously cheaper) lens of a digital compact camera can't work with light as well as the larger and more expensive, probably more complex, lens of a digital SLR size camera. Consequently the image quality possible is greater than with the digital compact camera. (Not to mention the other image enhancing qualities of a digital SLR like the size of the image sensor itself.)

If you want to take your photography seriously, get a digital SLR.

If you want your photography compact and convenient and relatively idiot-proof, get a digital compact.

Be honest with yourself about your approach to photography because there's no right answer, just the right one for you.