Not happening.
The Xserve was discontinued because no one gave a crap. Apple's software and hardware for the server market, whilst I'm sure very functional and powerful, have the disadvantages of not being scalable and being limited to one platform.. nay, one machine (for serious purposes). As of speaking Apple have two choices for OS X Server, either licence it out, or kill it, because right now it has no purpose for being.
The Mac Pro however has a target audience. Apple knows all too well the importance of third party developers after the 1980's (the Mac and it's GUI lost to DOS because the Mac had no software to speak of), because ultimately the success of an operating system depends on the applications that run on it. An operating system literally operates your system, as well as providing built in functionality it also must provide a strong development environment to be successful.
Now I'm not suggesting everyone needs a Mac Pro to publish an app like Caffeine or ActionPotato to the App Store, but big apps: Photoshop, Office, AutoCad, Mathematica, you're looking at a Mac Pro. If Apple want to keep picking up new marketshare, they need a lot of software on their system and they need a professional workstation for their developers. Hell even if they only sell 10,000 a year (made up number before anyone quotes me), those 10,000 machines are enabling the development of applications that bring in billions in Mac sales, and billions in iPhone and iPad sales.
So you certainly may see less hype around the Mac Pro, you might see Apple crank up the prices on it again, you might see less updates, but that machine is not going away because Apple are not fecking stupid.