I figure they couldn't get the LED fixture manufacturers to make the new lights compatible with the 6.6A system.

Did you get any of the insulators? In the hobby these would be the CD 125 Hemingray-15 and according to a fellow collector, clear CD 155 Hemingray-45 were used too.

Are they taking down the loops entirely or are they adapting them for 240/120V?

What they did here on the muni system was all the sections that had 2 conductors were modified to remove the "break" insulator by each light and the streetlight connection moved to use both conductors. Those sections that had 1 conductor were replaced with regular duplex secondary wire. The sections of 6.6A line still on crossarms are slowly but surely being replaced with duplex as well.

Nope the insulators were still on the lines, I couldn't reach them. The lines seemed to be motified in a way how you mentioned on the munis where you're at. Basically the whole entire city wide system was converted into 240v line wiring. All the old poles are still up though.

I'm still very surprised to know that that system is gone for good now. Especially, when I was a teenager living there, and seeing the entire city's street lighting system literally use it. Back then, I was naive in thinking that every city wired their street lights this way. I didn't realize that it was actually ancient out of date system still in use. They used a lot of glass insulators like the ones David Dahle mentioned and the modern porcelain ones were all dark blue in color! I'm glad I took many pictures during that one visit to Detroit back in 2014, when it was during it's last year in operation.