Paradoxes are impossible. That's why they're paradoxes. Altering the past is impossible because, if the past was altered, who is there to go back in time, not to mention they would have no motive to do so. The only legitimate way to finesse the existence of paradoxes would require really wild interpretations of quantum mechanics and general relativity. Stephen Baxter's Exultant made a stab at it, unsuccessfully in my opinion. Lost hasn't a prayer of a chance.
Making Desmond an exception of any sort has the same problem. I also would dislike lame "reasons" like surviving the destruction of the hatch. The storyline about the numbers preventing the end of the world has been detonated already. It was a dud. Revisiting it would just compound the error.
Genuinely plausible time travel without paradoxes would require intense planning. Which in my opinion is why Lost and Heroes are doomed to overall plots with no sense. And worse, no real dramatic validity. Slovenly time travel plots with paradoxes only work in my opinion in short form. Internally consistent time travel stories could work in long form, except for the practical difficulty of actually planning to make a long term story, and then actually getting someone to pay for filming it. Babylon 5 will likely remain unique.
Speaking of which---how many others have ever wondered where did the Chrysalis device orginally come from?
(I imagined a story line where the warrior caste wanted to create their competitive hybrids and built the original, which Ivanova then sent back in time, when she was head of the Rangers.)