I think you're analyzing it from the perspective of mathematical logic. In an isolated perspective, one employer (instead of all employers), can arbitrarily change the working time as 7am-4pm instead of 8am-5pm because it's mathematically equivalent to the +1 hour DST.

But society wants to coordinate on a bigger scale than that one employer you were able to convince. (Children's school hours, restaurants, tv schedules, customers calling help desks, etc). Also, the math viewpoint ignores the fact that there are long standing cultural anchors tied to specific times such as 9am-5pm[1].

The bottom line is that it's easier to address the coordination problem by hacking the time via permanent DST rather than adjusting everyone's familiarity around nominal times. Changing out the nominal times displayed in glass windows doesn't look easier than implementing DST. Even changing nominal times in the digital realm is a hassle: think of all the recurring meetings in MS Outlook, Google Calendar, iPhones reminders, etc that say 10am-11am. Millions of people would have to change those times to say 9am-10am. Again, leaving those nominal times the same and fixing it more "globally" via DST seems a lot simpler.

[1]https://www.google.com/search?q=store+opening+closing+times+...