So, if someone, somewhere has built a single wormhole on "opposite sides" of the universe, we're still done.
And how would your mechanism work? How would something get information that a wormhole had been built 20 million light years away, 10 thousand years ago?
And, I'm less than convinced that this would solve anything. Travelling through a series of wormholes would seem to be possible still, and cause problems.
And why is it such a big deal that there is a limit on the speed we can travel? Why waste time trying to come up with ways that cannot ever happen, instead of figuring out ways to live within our constraints and what that would entail?
The mechanism is space-time. Ie A wormhole is created through bending space-time... lets say that the act of bending space-time causes an instability in space-time which prevents other wormhole from being formed with in a certain radius. Ie. If space time is say made up of a plane of lego two blocks thick and a wormhole requires you to take one layer of blocks out of spacetime to make the wormhole (in a certain radius depending upon how far you want to go). This means you can't make another wormhole within that radius because you dont' have enough er ... lego bricks to do so.
And it's not absolutely a big deal, personally I think humanity needs to get off the planet (the earth is a cradle of the mind, but one cannot forever live in a cradle etc etc). Plus yellowstone, gamma ray bursts, asteroids, plagues, environmental catastrophes etc etc. But we dont' need FTL to do that, suspended animation, mind uploading to computers then sub-light travel would all work just fine.
Having said that i'm just unconvinced as to the finality of the FTL barrier. Remember yes Einstein created the theory or relativity which prevents FTL and the dude was a genius and he's generally right... but he also created the concept of a wormhole (that's why it's called an Einstein-Rosenberg bridge) so FTL or no FTL einstein is still right.
Oh and wormholes opposite sides of the universe and we're still done? Then well possibly yeah we might be. Of course there is another possibility which only wormholes which can cause causality violations are impossible in the universe so it might be allowable. Though i'll have to think of another metaphor rather then lego bricks.
(think of instability, or the surface tension of water and stuff like that)
Oh and again, travelling through a series of womrholes isn't possible Because you can't have two wormholes open together in near space. You'd have to travel at sub-light speeds from wormhole to wormhole. So a series would have to be so far apart that by the time you travelled from one to the other you'd no longer be able to cause causality concerns.