Well, no one else has taken a shot at predicting the future so I will. Our office made the switch to BIM, in ArchiCAD it is called Virtual Building, a few years ago. We were a PC office and looked at ArchiCAD, Revit, and Vectorworks as our main contenders. ultimately we decided that for our needs ArchiCAD worked best. It has a long, 20+ years, history and was obviously not just a flash in the pan. It communicated better with the other programs we needed. It handles text better than Revit (at least at the time, I am not sure if this is still true). And best of all it has this wonderful forum. Today, we are primarily a Mac office and are quite happy.
The bad new is that here in the US Autodesk again appears to be buying the market with free Revit upgrades and massive marketing, so ArchiCAD will remain a niche player, although hopefully with a bit bigger market than before. For us that was not a problem as we are a small office and have VERY stable staffing. For you...you must decide.
"Is ArchiCAD here to stay?" It appears to be from both it's history as well as the things you hear. However, like life, there is no guarantee. But they don't appear to be going away soon.
Poke around on the forum and you can begin to see some of the issues that surround the change to a BIM solution. With decent training, or if you are lucky enough to have someone who can champion the program, you should be fine whatever program you choose. Without that, the best program in the world will fail you completely.
If you have other detailed questions, you can PM me and I will answer what I can.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K