I believe arch viz will switch to unreal, when unreal engine 5 comes out you will see alot more switching. However i do believe their may be a shift within the archviz industry for artists to work for architect practices directly.
Due to unreals quick turn around for visuals/ animations and walkthroughs and the implementation of Lumin and Nanite, it will likely be used as a tool inhouse to help architects visualise there plans, more quickly and iterate alot more. I'm not sure how this holds for viz studios, but as long as your on the ball with it, it will be an easy transition. People are talking about lightmapping which is what you do now in unreal engine 4, but this will change completely come unreal 5. I think the process is going to be alot more streamlined for both games and arch viz.
another thing RTX cards will be superseeded by the 3080 giving over 2 times the power it currently has to run realtime. I think we will transition to doing the high quality we are used to inside unreal very soon.
On the other hand, Stills / photomontages / VVI's will always be needed i can't see them going away, but i think we all need to take ue5 seriously when it comes out. Epic does have a good grasp of what we need now and will implement it as they want our business