They might see the product by price and not age, but there’s a bit of an expectation when it comes to any consumer product. If what you’re buying is cheaper than the norm or the cheapest in the range, you typically expect less service or a shorter service period. If you buy a 10 year old car, you’re probably going to realize that it’s outside of its manufacturer’s warranty and that you might need to buy aftermarket parts to replace some parts that may fail. If you buy flash memory from Wish.com, you realize you’re probably not going to get any customer support. If, instead, you buy a $100,000 car or a $20,000 Swiss watch, you tend to expect better service and/or more longevity.
In the specific case of the person asking you about the Apple Watch Series 3, let’s say they asked you last Thursday instead of today. Would you have told them, “well, it’s a 5 year old design, they’re still selling it but it might only get one or two more OS updates, and you’d be losing out on a lot of features” or would you have told them “Apple’s still selling it, so, while it is a 5 year old design, you don’t need to worry about them dropping it”? Would you have even told them that it’s a 5 year old design? (If not shame on you, unless, of course, it’s someone you intensely dislike!

) And, when they asked you today, did you tell them, “it’s a 5 year old design, and they’re not going to support it in the next watchOS version”? The salespeople in the store would probably try to dissuade people from buying them, I’d imagine, and would instead point them to the Apple Watch SE, unless it’s someone who gives off the air of knowing what they’re doing.