Major model changes may be rolled in every few years, and it does take a couple of years to get from someone's idea to a "firm" concept (lots of committees have to agree). A typical time, once a concept is "committed" would be perhaps 3 years to get to where you have a "list of parts" that's specific enough to start placing orders. Allow 6 months to get enough parts to assemble test vehicles, and a year from the same starting point to place procurement contracts for parts - if you're ready to "commit to production."

If you're lucky, you test for about a year and you're ready to actually start building. (Things usually come out about even - if you're doing the math above - because some critical supplier will always be 6 months late with the first production parts.

A "major rev" design will generally be used for at least 5 years, with "cosmetic changes" to make "new models" so there isn't likely to be anything revolutionary for a while - especially if you buy a new one in the same model line as the one it replaces. A few brand names have carried the same "model base" for as much as 15 years or so.

No one can afford to make an "all new" car. One "new" concept may go on one line, while another "new" concept goes on a different line. The ones that work eventually are "migrated" into all lines.

Watching on a year-to-year basis, it may be difficult to see what is happening, but if you look at all-wheel antilock braking, high-energy ignition, all-passenger air bags, passive restraint belting, stratified charge combustion chambers, computer programmed manifold injection, all-wheel drive, stiffness controlled suspensions, non-$n$o$b$l$e$ catalyst systems, and even GPS navigation - all of which are in a significant number of "current year" vehicles - there has been a lot of change in the last 5 or so years.

At least three major producers will field test fleets of "hybrid" motor/battery vehicles within the next year - and will lose rather large amounts of money on them. At present, this is a "niche" concept, that may someday be ready for general use, but for now it doesn't work well enough for large numbers of people to pay for.

At least four separate and fundamentally different "hydrogen fuel" fleets have been "in the field" for at least 15 years, but if Congress mandated, as has been proposed, that you had to drive one, you'd be very unhappy with the usability of the vehicle.

At least two models from major builders will try out engines based on a new concept called "volume burn direct injection" this year. If you buy one, you won't know it's there - unless your mechanic's answer to everything he can't fix is "it's a new-fangled piece of sh...t" instead of following the manufacturer's service sheets.

Workmanship on individual vehicles? ... well now that's another question.

John