Why are they still able to sell what is effectively a 30 year old computer for as much or more today than when it came out? Because they managed to get the family informally standardized as "The calculator every teacher in America understands well enough to manage students who use it. Therefore pretty much everything else that could be as or more advanced is effect banned."
It was an amazing piece of kit when it first came out. No doubt you could make something 100x better and 10x cheaper today if someone really tried. But, they would fail commercially because you can't design-in 30 years of legacy in the US school system.
tests like SAT, ACT, and some AP exams are using Desmos, yes
however:
- this means you have to fiddle with a popover window and can't always see the full problem (especially when the reference sheet is also online)
- you have less muscle memory and often take longer
- harder to multitask (you use paper anyways, and the paper to calculator friction is lower than the paper to trackpad friction
- trackpads on school computers are usually worse, which compounds the problem
- some specific functions just don't exist
essentially using Desmos is like using a physical mouse/trackpad, while using your calculator is like using VIM motions and keyboard shortcuts with a concave split keyboard. it's technically more intuitive and can help in certain scenarios, but it's useful to have both.
this sounds trivial, but it's not, especially on tests where you have about or less than a minute per question
ideally you have both a handheld calculator and Desmos though