The doctors are only expressing what Malthus, http://en.wikipedia.org/wiki/Malthus, had to say about over-population. And look at the trouble he got into with his theories.
As for the ageing population issue, there's an argument that goes like this:
1. People are living longer after retirement;
2. Therefore, we need a larger workforce to generate taxes for pensions;
3. The native workforce is dwindling so we must increase immigration to help us meet the commitment to a decent pension for all.
There's a flaw in that logic, unfortunately. When this larger workforce eventually hits retirement we will need even more taxes to pay their pensions. So we will need to increase the size of the next generation of workers yet again, ad infinitum. I think we know where that scenario is leading. I believe people should be encouraged to work longer. There is a problem with that proposition of course because of the endemic ageism in society. You need look no further than IT for instances of such an attitude.
Going back to the population-size issue, I suppose the difficulty a lot of people have is conceiving of natural limits, not just to population but also to many other areas of human activity, such consumption. Petrol is a good example. This is a limited resource; most people concede this fact but proceed to consume it in the kind of way that suggests they actually believe it is limitless.
Personally, I don't see ever emerging the kind of global consensus we need to tackle over-population and over-consumption. It's a dog-eat-dog world. Our politicians will continue to sell us a dream of a better tomorrow because that is what we wish to hear. If they say otherwise then we dump them out of office.
For those who still believe over-population is not a problem for us I recommend watching events in West Africa. Distance-wise, it is very much on our European doorstep. There is as yet a tiny trickle of people heading for Europe in their effort to escape poverty. The damage to their environment can only accelerate this mass emigration.