I personally keep in my daily backpack a reusable Baggu that packs down nicely and doesn’t take up much space. As long as you have a daily bag/purse/whatever and you’re not strapped for space, it’s pretty easily solved, yeah. I live in not-NYC so I also keep a bag in the car.

I have some sympathy if you find yourself on a weekend out and about and needing to spontaneously buy some victuals and don’t have any bag on you - but that’s the cost of us having less litter.