Even so, in tennis, wrist surgeries which fully heal (or heal as far as any surgery allows), are generally less severe because wrists aren't as dependant in flexibility and movement as a shoulder, knee and hip. Contrary to what you seemingly believe, wrists aren't under as much strain as any other joint. All lower body joints are weight bearing and are dependant on movement, whereas the wrist is to stay locked the vast majority of the time. The most injury prone and serious upper body joint is the shoulder. Once you injure a shoulder as a tennis player, it's only going to be a matter of time before you're back in the operating theater. That's because the most range of movement occurs in the shoulder thus making it more prone to injury, which is a general rule for any joint.
Yes, it was a serious injury, but to constantly hear of this "career threatening injury" is nothing but hyperbole. Tennis players rarely suffer career threatening injuries and many have suffered wrist injuries for them to come back and never reoccur and then there's those like Canas who've had 4 wrist surgeries and have been able to play in between without problems.
Yeah, maybe it's a bit hyperbolic, but whenever a player goes through surgery there's always a chance he might never fully recover, so there was a risk. More than the injury itself, the fact that he basically lost two years of his career (one off the tour, the tour trying to come back) has hurt him a lot, at his age those were important development years, not to mention his rivals at the top of the game kept working on their game while he was sidelined.
I agree that it was not the worst injury ever or anything, but it definitely derailed his progress and put his career at the highest level at least at risk. But he seems to be bouncing back, that's what's important.
More than his serve, I'd rather have him improve his ROS
His ROS is alright. The only player whose serve he always seems unable to return properly is Federer, he just can't get a rid of his serve. With none of his younger opponents being a great server, improving his ROS isn't that prioritary, he can return Nole and Nadal's serve just fine.