I have an HL-2270DW and the toner low light comes on well before prints start washing out. Fortunately the toner life is tracked by a plastic gear on the cartridge, and it takes just a minute to roll it back. When it does run out, good third party replacements are under $20.
An alkaline AA may have more capacity, but the LiPo has lower internal resistance and it’s electrolyte is flammable. The worst the alkaline would do it shorted or punctured is get warm and poop out KOH.