Wake up, Zaper, saying 'It's supposed to happan in the plot' ain't gonna help, because it's not actually PROOF. Miguel CAN kill Serge if you're not careful enough and Lavos can kill Crono and co if they're not cautious enough. We can guess the world will still go on.
Now, Lynx could overpower Lucca because he's got power from FATE to backup-With a power like Forever Zero and a partner like Harle, the two of them could easily overpower Lucca. Serge can kill the Time Devourer (And note it being a very hard boss, unless you've finished the game a second time), but then again the Time Devourer isn't matured. You can't put the unmatured (and possibly incomplete) Time Devourer in the same level as Lavos- Lavos had millions of years to absorb DNA and power AND mature. The Time Devourer had, in the most, 13000 years, more than a houndred time less than Lavos, and it was umature.
Put things in perspective of their power-Miguel, for all his might, failed to beat Serge and co, just as Lucca failed to beat Lynx and Harle, and just as Lavos failed to beat Crono and co because they were OUTPOWERED AND OUTMATCHED, and not because the plot demands it.
