Even police departments don't send their paramilitary groups (eg. "SWAT") to every scene, police don't need to be at every emergency call. In fact, this is already the case -- when you call 911 for a medical emergency, an ambulance or medical fire department unit shows up (usually alone). In the same way, "defund the police" means use police more judiciously, not necessarily eliminate police.
> The future of America is that of a lawless gangland like Mexico if the police are "defunded".
Citation needed.
Gang violence in the US has been declining for more than 2 decades, even as the population grows and US police case closure rates are far below 50%. Police don't have as much effectiveness or as good of a reputation with innocent civilians in gang territory as I imagine you think. Also, a significant portion of the US already has and carries guns, so it's not like gang violence would just expand boundless even if there were no police.