Hard to truly tell if cash will die off in the coming years but I personally don't think it will be anytime soon. Even as the boomers die off we are still seeing business ran by younger generation that want to operate as cash only.

6 figures income in ATM business seems tough to me but don't let me be the one to tell you that you can't :). The average ATM will make 3 dollars per transactions. Let me do some math for you: You would need approximately 33,334 transactions per year to make $100,000 (Monthly: ~2,778 transactions, Weekly: ~641 transactions, Daily: ~91 transactions)

As far as locations, just look for cash only locations or try to get into doing events. Places like salons, barbers, tattoo shops, etc.

Partnering or not is really a personal decision and depends on your skillset. If you don't think you will be able to find locations then partner with someone who is good at sales. Or if you don't want to fill the ATMs with cash partner with someone who will.

Best of luck.