I don't understand why, if the 152/172 airframe is so good, they are not worthy of an engine/avionics upgrade. Shirley a complete stripdown, repaint, new carpets & busted trim replaced shouldn't cost more than a new unproven a/c?
Seems to me that for training in a cost sensitive market, cosmetically rough or clean aircraft do the job equally well. As Pilot DAR points out, there may even be advantages to keeping them 'rough and ready', so that's the way they stay. For an individual owner I think it makes a great deal of sense to refurbish them, and in the US it is the way some people are getting 'new' aircraft now. That is aided by many of the old airframes having been in individual ownership since day one, and therefore having relatively low hours. Two years ago I bought a 40 year old, two owner aircraft with 900 hrs TT and have been doing some of what you describe.
I think there are too many existing airframes to justify building new ones when the benefits of mass production don't really apply to new aircraft, and the labor content/cost for building a new airframe is far more than the (say) $30K for which you can buy an existing plane.
Sadly no one seems to be capable of designing what is effectively a airframe as strong as the C150/2 /172, PA28/38 and mate it with a 10% ethanol sipping EFI rotax. But as soon as they do I suspect the current training fleet will be replaced within a few years.
The issue there is that the Rotax really doesn't make enough power. Resistance to mishandling (ruggedness if you will) takes weight to achieve, and an 1100 lb airframe needs 125 HP or so if you want it to carry two 2013-scale people and 2013-required equipment. The rugged Rotax powered aircraft ends up with lots of wing area, Cub level performance, and still costs a lot of money that has to paid back. I think if renters and students valued new-build airframes over performance then a ruggedly built Rotax powered trainer could make sense at the same wet cost/per hour, with the same airframe life. Just my POV