"At the same time, there is no financial pressure to lower the prices of its unchanged and rather low-tech graphing calculators since consumers are happy to keep buying them."
"For those reasons it seems that TI has little incentive to alter the basic design and functionality of its popular graphing calculator products"
How about looking at it from the teacher's POV? If they created an entire classroom lesson plan (including handouts and worksheets) that expect the calculator to function a certain way and then TI changes the interface, what would teachers do?
What would you do? Rework all of your materials, or go find another calculator that mimics the old interface?
TI-83/84 are good calculators - even here at HP you see a lot of them although the enlightened insist on RPN.
I think a large part of the success is that high-schools pretty much insist on a TI calculator. I bought my daughter's 84 on craigslist from a college freshman - I thought $120 for a new one was a bit steep.
Consumers are required to keep buying them.