O5e works at higher levels, really well IMO, and I assume with all the extra work put into it that A5e works even better.
PS my group is lvl 15 in O5e now and having a great time and I find it very easy to DM. But I have also DM'd D&D for a long time.
I'm not sure what you mean by "left to themselves" as there is ton of DM content in 5e. And since A5e builds on O5e, even more for A5e. Do you care to clarify what you mean?
FYI, there are resources in the DMG that allow you to run pretty much run any encounter with just a sheet worth of easy rules/tables. I use these all the time to improv encounters and it works really well for me. I will admit that these are not in all one convenient place in the DMG though.