Depends on where you will be abandoning them. Street cats usually didn't live long tho. Infections derivated from food poisoning, injuries from fights with other feral animals or from the general urban environment, parasites, urban predators like falcons and dogs, starvation or roadkills are usually the fate that awaits stray cats a couple years after being abandoned, and they are usually just nasty, painful and brutal. Also stray cats represents a great risk for other species like birds, small mamals and reptillians. That said I still support mercy kills over abandoning them (not with rat poison, that stuff kills slowly).