I'm no early bird, but I can still keep a schedule if I must. I suffered years of waking up at stupid o'clock for public school, frequently sleep deprived, and that was back when I only knew about caffeine for helping to counteract sleepiness... These days I'm fortunate enough to be working for a geographically distributed team. I'm salaried. I can work from home when I want (and did almost all the time for 2 of my 3.5 years).
I generally come into the office around 10:30am, though sometimes it'll be 8am. I wouldn't mind coming in around 2pm and working into the evening, but I frequently need to work with other employees who have a more normal schedule, and I have an 11am standup so I have to be awake by then anyway. When I know I need to go to sleep, I pop a melatonin tablet and put on a sleep mask to block out light. When I first started, I forced myself to bed before 11 each night and made sure to be at the office before 9 each morning, until I observed that people didn't care about strict schedules.
There's an understanding that as long as I make the meetings I agree to (some are in person and require travel) and management is happy with my output, my schedule can be quite flexible. Some weeks will be less than 40 hours, some weeks will be > 50 because of some deep bug I just want to get over with. Some days I'll spend a bit too much time on HN or taking breaks to the snack kitchen, other days I'll sit down when I arrive in the morning and not stand again until late at night due to focus. It all tends to balance out. In any case when the lines aren't drawn clearly it's important to find out at what point the fuzzy boundary becomes solid. This "cultural shift" that OP was complaining about is just the new generation of employees finding out where the hard borders actually are, and discovering that at many firms they're a lot further back than they used to be and they can be pushed even further with a bit of pressure. I'm waiting to see someone show up to work in a fursuit, and wish there were an old-timer suited IBM employee there to witness it too. Many firms have realized that to a large extent, for many jobs what you wear simply doesn't matter.
At a previous job, I also had a pretty flexible schedule, but we tried to keep it "between 10 and 11" yet I kept coming in closer and closer to 11 all the time until a few times I was in after 11 and more than once I overslept and came in past noon. We changed it to "be here by 10 or we might have to cut ties". No further problems.