With the return of overly formal and distant/avoidant social relations, people feel less comfortable addressing strangers in friendly ways.
You still hear "dude" and "man" among youngish males, but even there it's not as common as it used to be. "Dude" is now more of an interjection, like "Wow!" Hence, "Dude! You'll never believe what I just saw..." It expresses the speaker's feelings rather than address another person in an informal way, a la "What's up dude?" "Not much, dude. How 'bout you dude?" (No one claimed that informality would make you sound smarter.) It seems like "man" is the only one left, like a fast food worker who gives you your order with a "Here you go, man." "Cool. Thanks, man."
The change that really rubs me the wrong way, however, is the ubiquity of "Sir" -- like, I'm not royalty, I'm not a potential employer who you're petitioning, and I'm not one of your superiors whose butt you have to kiss. It's not only retail workers who are required to say this -- it's panhandlers (who have no boss giving them orders) and everyday people on the train or other public spaces. In the retail setting, their bosses aren't just giving them pointless orders to address customers more formally -- it's likely because today's customers expect that level of formality from the workers.
I wish it were just a sign that I'm considered an old geezer by the young 'uns, but they all think I'm in my early or mid 20s, and besides, they address one another that way too. Generally girls call guys their own age "Sir" in the right context, while guys tend to reserve "Sir" for older men.
I'd be fine with no address at all in the retail setting, which is where you hear it most often. "Is there anything I can help you find?" -- I don't need to hear "Sir?" at the end. "Sir" makes it feel more like a patron-client relationship, rather than them being there to help out another person like themselves. Again, I don't blame the workers so much, although I'm sure that as members of today's world, they too prefer that kind of detachment rather than talking to strangers in a friendly way. It's due more to distant customers these days wanting more, well, distance between themselves and the workers.
If they're going to use some kind of address, what ever happened to "Mister"? That sounded more informal because people used it not only in a polite context -- "Need any help carrying those bags, Mister?" -- but also in a talking-down or putting-in-their-place context -- "And just where do you think you're going, Mister?" It was a leveling form of address, not used only toward superiors. Even when it was used toward elders or superiors, it just sounds more like someone you know from the neighborhood, like "Mr. Callahan from next door." It made young people sound less groveling and pathetic when they addressed their elders, and didn't make the elders feel like distant old fogies who needed to be carefully addressed in order to protect their ego.
Plus, "Mister" allowed all manner of ironic usages among folks who already knew each other. One friend tries to back out of a commitment to another one, and he replies jokingly but seriously with, "Woah, guess again, Mister. C'mon, there'll be girls there -- it'll be cool." It's like a friendly little poke in the ribs.
Or a girlfriend might refer to her boyfriend playfully as though he were a stranger. Remember at the end of Back to the Future, when Marty McFly is trying to appreciate what a sweet ride his new black truck is? His girlfriend Jennifer walks up behind him, pauses, and teasingly asks, "...How 'bout a ride, Mister?" It's such a warm way to flirt because "Mister" calls subtle attention to the fact that the two of you are in fact already close to each other.
And of course, sometimes the usage was intentionally underscoring the fact that the two were total strangers -- as in, "strangers in the night," or "relying on the kindness of strangers." Like some babe asking for a ride home: "You goin' my way, Mister?" Or when she recognizes that you're new to the nightclub or bar: "Say Mister, don't believe I've seen you around here before..." It's acknowledging the lack of a former acquaintance, while treating it as no big deal, nothing that we ought to let stand in the way of hitch-hiking, dancing, or what have you.
It really was a versatile form of address, and it'll be a sign of changing times once we start to hear it (or something like it) in common speech again. Until then, for God's sake, don't call me "Sir." It takes you right out of the moment.