I have this happen all the time when I look at my friends' bikes and think out loud. "Wow, that BB is smoked, doesn't the squeaking bother you?" "That's a pretty good road bike, but you never go offroad, why do you have big nubby cross tires?" "dude, only half your brake pad is touching the rim, the thing has an L worn into it." "How do you even brake with the cable housing split like that?"

Bikes are like sneakers in most people's minds. Imagine if you walked into a shoe store just to buy a new set of laces and the salesman told you (with genuine concern for you) "man, you know, those have classic overpronation wear, the longer you wear them the more you are messing up your feet and knees. Those are really the wrong shoe for you anyway, that's a trail runner and you're wearing them on pavement 99% of the time. And man, the treads are shot, that's really dangerous, some day you're gonna slip and take a bad fall, maybe in front of a car or bus." Ok, maybe one of us who is into how stuff works would get into like an hour long conversation about shoes and foot anatomy and sizing and stuff and really make both our and the shoe guy's day, but most people are just going to be like, "Look dude, they're my shoes, they got me in here, and as soon as you sell me the laces I came here for they are going to get me right back out again."

There's not much you can do about it besides break it to them gently and do everything you can to prove you're not just trying to take their money, even if it just means doing a $20 triage job today and hoping they come back for what it really needs.