Why do you think they had their injuries? Were they self-inflicted or what?
What does it matter, you can't infer there will be no new players like those two. BTW, people have knee and hip problems without being tall or playing a taxing game. Conversely, Goran was as high as Safin and never suffered knee problems, just the shoulder, so the implication that you know the reason behind those injuries is also faulty.
Lets wait for them to go a couple of years without major injuries and win some Slams. Then we talk.
Winning slams is a tall task, you can't expect every top player to do it. Being in top 10 and making slam semis, for instance, should be more than enough to say they can obviously compete.