What makes you think they work less hard?
What do they do with all that money? I see the following options and outcomes:
1) They stick it in a mattress. Okay, that is not really going to help anything and no one (of any note) is doing that.
2) They buy stuff. Well everything they buy was built or created by someone. They buy a nice house. How many people had jobs because of that house being built? They buy appliances for the house. Again jobs. They pay for landscaping. Again jobs. People invest in the companies that produced those items and get stock dividends or can sell the stocks more a profit. Maybe the doc hires a nanny.
3) They put the money in the bank in a simple savings account. (Not a great investment, but still some money will be in there). That money is used for loans for others to buy cars, houses or start or expand a business. Plus the people who work at the bank are getting paid.
4) They invest either in stocks, mutual funds or IRAs. See above for where that money goes as well.
Oh and that MD whose practice is making more money will likely hire more staff to meet the needs.
Also, taxes change behaviors. I do not really buy the idea that once someone reaches whatever the plateau is that they will stop working at least not on a large scale. I do know of one instance where someone I know was offered a contract job and realized that if she took it, her net would not be all that much (it bumped that money into the higher rate) and decided that it was not worth it; she preferred to vacation with her family (opportunity costs, after all).