There's already a solution in place to handle cashless tips for them. They aren't responsible for the transaction fee a lot of processors pass onto the gig economy. It works relatively well, even in the event of loss of internet or power.
The laws of averages work out for an average server, I worked for 5 years in the restaurant industry and two years of that was 100% waiting tables. I averaged $24-$26K between those two years in two different states and 4 different cities. Both of those states are at the US minimum wage of $2.13/hr (for restaurant waiters).
I worked about 30-40 hours a week. We're talking ~$12-$13/hr average. It's was definitely much better than any minimum wage position I could have had.
Anecdotally, the servers I knew who complained most about the wage were also the least responsible ones with their money. We worked at a restaurant that gave you your tips at the end of the night and to many peers, the urge to spend that cold hard cash was too strong. Those responsible but just underpaid were underpaid either way; they still could usually keep it together at $12-$13/hr, still a good wage for someone relatively just getting started career-wise.
Whatever wage you made from the $2.13/hr usually just covered your taxes. Most of my deposited paychecks were less than $10 every other week.
I know that this isn't what other countries do, but it does work and keeps most families and individuals out of the poverty level. The requirements for work are relatively undemanding and most people who want a job serving get a job serving.