This is something that the young simply do not get, when you are young you are as healthy and as energetic as you are going to be, but they believe due to their inexperience that this is always going to be the case and they are mistaken, so they need to take advantage of that energy and time they have to try to build something out of themselves, that way once they are old they will be able to reduce their workload and still be in a good position.
I think most young professionals were just enjoying the things that they can afford while they still have the energy to do so, it's not that because they lack in experience. Working too much while you're still young would make you miss some things you can't do when you're older. I'm quite sure young people knew they need to work hard, though they don't have any idea how stressful life could be when you are in the adulting phase, that you'd even lost track on your work-life balance. Also, when you get older being employed in company, I don't believe your workload are reducing. Yeah there are promotions, but higher position means additional responsibility, so i don't think work load will be reduced in that case.
I still think it is better for those that are young to work hard on their youth and be frugal, will they miss some experiences if they do this? You can bet they will, however not everyone is keen on working until they are 60 or 70 and then drop dead a few years later, someone that was smart with their money early on and tried to maximize their income could easily retire at 50 or even 40 if they are smart, and then they will have a lot of time and health to enjoy the rest of their lives.