I think it comes down volume of riders.
Closed roads allow you to get a greater volume of riders on an event.
Without closed roads, even with spreading out the start times anything approaching 1,000 riders will be become a problem on the roads.
My first (open road) event was the 2013 Tour o' the Borders with biblical rain. One of the problems on that ride was the oncoming cyclists on Paddyslacks who had decided to pack it in - I didn't know that was an option...
My first (closed road) event was the Etape Caledonia where I was placed in the last group to set off as I didn't guess my speed correctly and I ended up overtaking 2,000 people before I finished. That was a lot of fun :-)
Each have their place in the calendar.
I think people can be justifiably peeved if the roads are closed but the volume of riders don't necessitate it. Etape Pennines being one such event that only lasted a couple of years.