Makes me wonder why thermal US Letter sized printers are not more popular? There are some portable ones I can find at quick glance but not many desktop versions. Thermal printers do not require ink and most things you print are in black and white anyway.
Are there downsides to thermal printers I don't know about?
It depends on whether what you are printing is intended to last, and for how long. A thermally printed receipt in a car in Phoenix during the summer might last a few days before becoming unreadable. I have Ticketmaster receipts from around 2000, that are unfortunately fading and will eventually be blank tickets again.
"US Letter" is a horrible paper size - what is it with the U.S. and ridiculous measurements?
ISO 216 is the international standard (e.g. A4 size) and because they use an aspect ratio of the square root of 2, you can fold in half one size to get the next smaller size.
Thermal printers tend to be expensive as you use either special paper or special printing ribbons. They quite often create fumes which isn't pleasant and the prints tend to fade after a while, so not much good for archiving.
Sure. The prints fade quickly, especially in the sun. And they are susceptible to heat, rubbing your fingernail along is enough friction heating to cause a black line. So they are not all that usable for something more permanent than a burger receipt.
Are there downsides to thermal printers I don't know about?