A lot of people didn't like the substance of psych, until they realized that psych was a lower paid lifestyle specialty, low overhead and you can still own your own practice, and make decent money for what you do.
EM is shytty, in that if you take away the compensation the schedule and the BS aren't worth it anymore.
In all this, forget the people born to do it, who would do it practically for free, they exist but aren't the numbers that make these fields competitive.
In any case, with EM and psych even money aside, you can see what draws people to the work and the schedule. In EM it ballooned the field. I can't speak for psych but they're a ways off saturation, especially if they can keep more mid-levels out. IM doesn't have the money, schedule, and midlevels can't move in the same way.
What does IM have going for it to make it the new lifestyle specialty grads flock to? Even with FMGs a lot of programs are sweatshops that never fill. No shortage of work for residents to do nationwide.
IM doesn't have the schedule or the money. I'm not saying it doesn't have the money or the schedule enough now to have enough interest to make some grads to want to do it at all, but IM is a long way off having what it needs money or lifestyle-wise to make it attractive enough to have any glut of providers, and as pointed out unless we overhaul the entire payment system that won't happen. And as pointed out, the usefulness of mid-levels is severely limited at that level of acuity.
And if a glut ever happened to make the money go down from what it is, those docs would get into some other practice, and they can. I've known internists that completely left medicine when they felt like hospitalist or clinic sucked bad enough.
EM and psych are more appealing than IM when it comes down to it, and that drives apps, which in turn drives income.
Look at the NHS. If hospitalist salaries go down too much, people just quit. And no one else can pick up that particular slack. It's like if gen surg stopped cutting. You can't have just anyone do it. Someone has to be top of some fields to even do what is being done.
IM has a certain soul suck to it that it won't get a glut, and salaries can only fall so far before you have to raise them to get bodies.
I think one of the big issues trying to have midlevels in IM, is that they just can't manage the complexity and the number of issues of the average train wreck safely or timely because they lack the training, and the amount of work they do, and the hours they want, and the salary they want, you have to hire like 3 midlevels for one hospitalist and you don't come out ahead in that scenario. As soon as a patient becomes complicated a midlevel can no longer deliver the efficiency of a physician. Factor in benefits and employer taxes and they're more expensive for what they can produce.