Where are you basing these magic assumptions off of? While the die shrink and 3d transistors do increase performance, the thermal envelope won't recede in laptops. The rule of thumb is that a laptop will always push the thermal envelope, so as to get closer to desktop performance. The effect is having the same thermal envelope, with a 5-10% (numbers based upon transistor count increase) increase in overall capacity.
There are no magic assumptions there, it is industry standard logic. Yes, Ivy Bridge will still push the thermal envelope, Mobile Sandy Bridge processor go up to 55w. However, there are also 45w, 35w, 25w, and 17w processors. There will be Ivy Bridge processor at 55w as well, but they will perform better than the 55w Sandy Bridge. Likewise there will be 45w, 35w, 25w, and 17w Ivy Bridge processors as well. What does that mean? Well it means you can likely take a 45w Ivy Bridge and replace your 55w Sandy Bridge and get the same or better performance(heck, you might even be able to go to a 35w and get the same performance, we don't know yet). Either way, Ivy Bridge will consume a lot less power when performance is the same, or it will consume the same amount of power with greater performance.
On topic, when have laptops even been upgradeable? The engineering challenges of fitting everything into such a small profile are huge, let alone making parts interchangeable. I'm still rocking a core 2 quad in a laptop, that suits its purpose well. It doesn't play the latest games, but it can still do 3d rendering and engineering calculations well (despite being 4+ years old). Don't hawk what isn't broken...
Laptops have been upgradeable for as long as I can remember, as long as you are willing to take them apart to do it. Over the years the process has been made easier in a lot of models, some only require removing the keyboard. I've got a Pentium III laptop that I upgrade the processor on that I bought in 2001, had a Celeron 900 and I put a Pentium III 1200 in it. And that is a perfect example of what will happen with the Sandy to Ivy transition, the Pentium III actually consumed less power due to a die shirnk, and performed way better. Some processor packages are soldered onto the motherboard and aren't upgradeable, but if you get a socketted one you can upgrade it, and most are socketted at this point.
+ Ivy will have a lot lower power consumption.
