I think you've hit the nail on the head there, Pete. As you say, not everyone is up to loco building, whether from kits or scratch (although for many this is because they've never tried. We all started somewhere - I learned my basic soldering skills on a Connoiseur Jinty because there was no RTR around anyway). Personally I get great satisfaction out of building any kit, and although I complain about poor instructions and bad design along with everyone else it is those very kits which give me the greatest learning experience and satisfaction when (eventually) completed. They are, of course, also the very kits which will dissuade newcomers from trying a second time.
You are also right about having good quality, reasonably priced but not necessarily Rolls Royce quality RTR so that new comers can join the mad house at an easy entry level. Such RTR can always be super detailed later if so wished - look at the pages of this very site. I take as a recent yardstick the Lionheart Pannier Tank and Ixion industrials and it seems that the Heljan diesels suit those of an infernal combustion engine bent.
I take issue with you over the cost, though. Of course, if you are going to buy a RTR loco every week or month it's hugely expensive. Some of the RTR stuff is, as you say, expensive in it's own right, and I'm sure we recognise that everyone's financial standing and financial ability to satisfy their cravings is unique. However, if you are able to build it's actually a reasonably inexpensive hobby. Clearly the time taken to build a loco, wagon or coach spreads the cost over a period of time. Compare the costs of modelling in this way with the cost of joining a golf club and associated green fees. Or a season ticket for a Premier League Soccer team. (And don't get me started about the costs of top flight rugby union or cricket nowadays). Model railways are, for some reason, regarded as a rather quirky hobby but I have rarely heard anyone question the spending of a couple of £thousand on an Arsenal season ticket. Your own well publicised enthusiasm, Pete, along with a few other notable figures helps to put this right, but in what other hobby are the combinations of art (scenery and "sense of place"), engineering and science (I'm including electrickery and electronics here) so well combined?
Frankly I reckon we're pretty well served by the trade, whether for RTR, kits or parts, and there's always the second hand market. I have to say that my own, personal knowledge has expanded exponentially since joining this forum, and it is a shame that more people don't take advantage of such facilities which are freely available.
Maybe I "doth protest too much" but I'll continue to buy the occasional (and inexpensive!) RTR for immediate gratification and build from my extensive collection of kits (which, as you say, will never all get built) to satisfy my creative needs.
Brian