I'm sure Dennis and others could give more examples, but here are a few links to US layouts that demonstrate there's plenty of "serious" modelling "out there".
Rensselaer Model Railroad Society's
There's a long video (somewhere on the web) showing this massive layout off to good effect.
Bruce Friedman's Website
UTAH COLORADO WESTERN
Appalachian and Ohio
Playing trains and having fun isn't confined to playing with Train sets.
"Serious" modelling can also be GREAT FUN.
When it comes to who owns a Train set or who is "into" "adult" railway modelling, here in the UK the demographics have changed significantly.
There has always been two major divides, childrens toys and railway modelling. The latter is a VERY diverse hobby. There is of course a middle ground between the two, often a transition phase, but some adults prefer to stay with Train sets, even in a "modelling context".
Horses for courses.... whatever floats your boat ....etc, etc.
Today, it seems the UK market for selling Train sets for children has shrunk so much that it has become rather obscure; whereas the adult market has blossomed, peaked and is now rather stagnant.
Recent affluence has seen adult spending on RTR and model railway products at an all time high, but the future (according to Hornby) is an uncertain and gradually shrinking market, and to me seems rather bleak. It's for this reason Hornby are spreading their wings and looking at European and Worldwide business (eg, Lima, Electrotren etc).
DCC will give the RTR manufacturers a major sales boost right across the range from Trainset to Railway modelling (All consumer products need freshening up or updating from time to time!).
The Hornby Trackplan book serves a purpose in that it encourages customers to enlarge and grow their own layouts; and in doing so buy more products from Hornby.
If Hornby are wise, the range of suggested layouts would cover the most simple to fairly ambitious, but in essence all of these are of the Train set type.
In my view there isn't any need to go futher, this book is aimed at that market after all.
Although it's not for me, I've no problem with that?