I would disagree with the above criticism of Lego. Most of the technic sets do provide more than one model that can be made, and once you have more than one set the possibilities of mixing and matching open up even more. My own three Sons were pretty well brought up on Lego and now all three have engineering degrees, with two of them working for Google. One of them, at about age 10, worked out how to make a differential from just the gears on hand, after I had explained to them how a differential works. So for a reward I had to buy him a set that had the actual Lego parts to make a much smaller diff!
I would point out that Airfix kits very definitely only make one thing! I won't get into arguments about the relative merits of Meccano and Lego, I think they are both wonderful. However, when my Boys were of the right age, Meccano was pretty much unknown in the shops here. Of course, now that they have left home, I get to play with all the sets myself, I guess at least until they get around to providing grandkids and they get old enough to make use of it all.
I would dearly love to log into the Lego ideas site and vote for the traction engine, however the login setup is totally borked, it thinks I am trying to set up a new account when actually I was trying to do a password and username recovery. Not up to the usual Lego quality standard!
John