I have been subjected in the past to claims made by purported "experts" in a number hobbies who claim to have absolute knowledge that cannot be disputed - they often misrepresent the truth. Your LBS "salesman" (either man or woman) is full of BS and flying around looking for pidgeons as customers. He tried to get you to buy a Aluminbium frame that is a fashionable fad without asking what you really want or need.
Any good salesman should first ask how you ride, where you ride, and what are you trying to achieve with bicycle frame and parts selection (racing/touring) - i.e. fitted size, gear ratios, comfort, weight, durability, parts availability, warranty, price, etc. and your salesman did not even ask these key questions. After collecting these basic facts, he should then propose a solution THAT THEY MAY OR MAY NOT CARRY IN STOCK. Too many dealers only try to sell what they have not what the customer wants or needs! It sounds like The LBS person tried to interject his biased opinions without fully understanding or acknowledging your needs regarding bicycling. He apparently thought you would believe he is a purported "expert" as he tried to lead you down his path to a quick sale. I am often amazed at how LBS salemen don't cover the basics of selling by asking the customer more questions and making a customer satisfied.
I now avoid one of these bozos here in Cincinnati who happens to own a campus bicycle shop near a local University. He tries to explain that I would be more happy with a Aluminium frame compared to Steel by saving weight and energy for longer rides. He claims that I can't get any older Campy parts anymore (ignore eBay or quality vintage dealers) and that I would be better satisfied with a Aluminum frame. He never bothered to explained the reality that most manufacturer bike components models now change yearly and their replacement parts are practically non-existant. His concept of part replacement is to buy a new complete assembly that may not match the original and skip the parts. He glossed over my detailed questions about the basics of bicycle selection mentioned above because he assumes that "newer is automatically better". The bottom line is that he tried to sell me a sales line as your LBS tried to sell one to you.
However, I find that cycling is far more of a mental sport where the rider with the most stamina and drive actively competes and wins - regardless of the minor differences of weight of the bikes. For example, if physical weight differences mean so much, why doesn't the Tour de France have a 15% overall decrease in time and increase in speed from 20 years ago compared to todays times? The answer is there do not seem to be large differences in TDF times for similar courses because a racers heart and drive matter more than the subtle weight differences of the equipment he is riding. I do not claim to be TDF caliber that my LBS tried to get me to think I am (far from it), however I am already satisfied with what I already have. Prehaps I need more steel to sweat a little more anyway to lose a little weight... <GRIN>
For those who claim that Aluminum bikes are "better" (whatever that is supposed to mean) lets see which frames last the next 20+ years... Aluminum frames that do not stand up to road stress well for the long term - compared to steel frames that are time and race proven for decades. I doubt if carbon or aluminum frames made today will have the same strength or rigidity like steel in 10 or 20 years. If I remember correctly, carbon frames deteriorates with time, heat, and other environment conditions. Steel tube repairs can be replaced with quality care and workmanship - how about Aluminum? Did the LBS salesman also explain that repairing aluminum frames is an absolute headache that is expensive and hard to repair? In this "disposable" world where everything can be replaced, I believe he was trying to sell you a disposable bike where you could throw away its purchase and use for a few years.
Prehaps he considers modern bikes to be frequent purchases that are considered "fashionable" rather than my idea of "functional". Professional racers often use Aluminium frames because their sponsors use them for advertising and pay royally for their expenses with new equipment every year, however I choose to ride vintage frames I have ( Raleigh Competitions, Professionals, and Team Pro) because I am interested in the ride itself, not in trying to copy someone else's idea of "fashion" and what a "Pro" should look like.
Regards, Steve "Satisfied in Cincinnati" Neago aka Questor1