The answer given to the headline question in the article is... "The evidence is mixed—and the practice varies widely across the world"

"Tipping also passes a chunk of risk from managers to servers, especially when between 20% and 60% of a waiter’s income may be in tips, as is often the case in America"

That number seems low, or at least in my outdated experience, it's low. I spent most of the 90s working in restaurants and tips were probably 150% of my income. I was paid very little per hour, and made quite a bit in tips. I then had to claim tips, which was always a smaller number than I actually made. I was paid $3 something an hour, made like $50 an hour in tips, and claimed like $20 in tips. Maybe that's not how it works now?