It's depressing to me that the closest I can get to the TI-92+ I had in college is to emulate a TI-92+ on my computer or Android devices (actually on a tablet it's not too bad). My other options (Octave, Matlab, etc.) are way too heavy for what I want to do and require too much time to learn.

They don't have that sort of pickup-and-use feature my old calculators used to have. I think it's the presence of most of the buttons I'm ever going to be interested in on the keyboard rather than having to figure out how to type something that means the same as an integral or some such. The discoverability of those buttons beats most modern software hands down.

I also suspect that writing simple to use general purpose solvers and simplifiers for the things the TI calcs do (the CAS) is likely very hard. I remember putting even pretty complex physics equations with all sorts of trig functions and exponents and integrals and such in during college and it handled most of them without trouble and pretty printed the result.