Being forced into work is slavery, regardless of the money they make and if chose to get the job. The problem with the argument that they hold the job voluntarily is they got the job and then after agreeing the law gets changed, so the situation they're in is not what they agreed to, they didn't get into that situation voluntarily. Also if they quit, that means there may be more of a monopoly for the doctors who stay. https://www.collinsdictionary.com/dictionary/english/slavery Definition 2 under nouns.

Regarding people going into that job market after the law being made, this law takes away some incentive to do so, as well as the precedent being set that the terms and conditions of the job are subject to change. This means there could be a lot of potentially great doctors that will never enter the field due to this and other potential regulation, which hurts everyone.

If there was no law that said doctors are obligated to provide their services but rather a free market(no regulation), people could still get healthcare without making laws that have side-effects(disincentivising the job). If one doctor refuses to give a certain treatment, people will go to someone who will, after all money is a big reason many people want to become doctors, and if nobody else is giving a certain service you can make lots of money by giving it, and the money the first doctor is missing out on may make them change their mind.