Bill, opinions and attitudes will be varied on this sort of discussion. What serves one very well becomes their own personal standard of excellence.Their buddy might feel their effort is off track for what he sees as the ultimate, and yet these two friends thoroughly enjoy sharing time on each others railroad. Neither is critical of his buddies efforts.
Funny thing, I didn't even know about 3 rail O scale since I figured all that was the Lionel toy trains I left behind as a schoolboy in the 1950's when I started chasing HO trains for the realism they offered. I started my O scale railroad using the only track I recognized, 2 rail Track!
I have to say, as far as P48 being the ultimate, well maybe from the potential is has to display realistic track and wheel profiles that has an element of truth. However laying fine standards of trackwork, and equipping extremely well detailed rolling stock and locomotives with fine scale wheels is really only part of acheiving the so-called ultimate model railroad. If an individual cannot build the total railroad, the very convincing scenery, exceptionally realistic ,structures and etc, the layout is no more the ultimate than if they used Atlas flex track, and RTR wheelsets. In the hands of a seasoned model railroader with diverse skill sets it can be exceptional nevertheless. In the mean time the guy using a RTR supply line is having just as much fun as anyone else, until enough diverse exposure opens their eyes to the potential for them to excell.
Don't know what the so called sweet spot might be all things considered. I personally build everything in a sort of rotation that avoids monotony, Might work on an engine for a couple months, do some scenery for a while, lay some track, scratchbuild a warehouse, monkey with my DCC and do some decoder installs and programing, I grow my railroad a little at a time and allow it to evolve as circumstances change. The original ideas I had for track plans has changed several times, and my operations are the better for it than if I had said ok this is the plan! Now im laying all the track, then installing all the electrics, then plant all the trees, etc, you get the point. This is my way, my sweet spot, and I don't pretend it must be someone elses necessarily. Do what pleases you.
I am also familiar with those guys who talk a big game, but their efforts languish month in an month out as they plan the dream, and never quite get going...no sweet spot there, better to do something and learn and grow in the hobby, the sweet spot will become known to those who make such efforts.
Bottom line, to anyone who wants a model railroad, get busy, learn as you go, make mistakes, change as necessary, always have a project or two going on, don't get bogged down in the details so much that you get totally stalled out, and what ever you do, don't become the famous armchair model railroader...he ain't getting anywhere sitting on his dead spot!
Bob