I originally worked with AC back in '92 - '93 and loved it -- moved back to the mainland (from Hawaii), and everything here in Denver was AutoCAD. The firm I worked for in 2002 was looking to transition to ADT, and I knew that even the AC of 10 years prior made ADT out to be a joke. Of course, the idea of changing CAD software fell on deaf ears.
A group at the firm I'm with now had a similar experience when presenting AC to the partners at about the same time. Then AutoDesk purchased Revit and "BIM" became mainstream...
I was given the task of evaluating BIM software at the beginning of 2005, and it soon became clear the choice was between AC(9) and Revit(7). We ultimately chose AC because:
1) AC had over 4x the market history as Revit, and there was simply more information available: third party applications, object libraries, tutorials, external support systems and publications.
4) The Lightworks rendering engine
5) We felt AC far more flexible, without Revit's "enforced associations between elements" and automatic updating
6) Not having to deal with AutoDesk
7) Of lesser importance, but still considered: National CAD Standard compliance, plug-in for SketchUp and the fact that AC is cross-platform
Now, a year-and-a-half later, Revit has made significant progress (although, I'm not sure I'd want to manage a system that upgrades every 6 months), but I still feel AC superior. I believe the competition in the marketplace is doing AC (and us) much good, and eventually the two systems will be comparable. So... it really comes down to which software you're the most comfortable with...
I don't know if this answers your question, but if AC is the best BIM package on the market today, why change?
MacBook Pro 2.4 GHz Intel Core i9, 32 GB of RAM
AC25 US (3011) AC24 on Mac OS Catalina 10.15.7