Well, when he decided to stick around we didn't have the Krakoa Resurrection Protocols McGuffin to magically bring him back. So when he decided to stay it absolutely would have broken the timestream had something happened to him.
Because he's not supposed to be in this era, at this time and at this age. If he were to die, just like if one of the O5 were to die when they were in this time period, it undoes all the things they've done, right?
Well, remember all the things older Cable did in the past? Taking the New Mutants under his wing and forging them into X-Force? All the battles that team has fought? More recently rescuing and raising Hope when mutants had dwindled down to an endangered species after M-Day, and bringing her back to the present in Second Coming so she could restart the mutant gene? Just like if Teen Iceman or Cyclops died here and now, before going back and being mindwiped, it would wreck the timestream; if Kid Cable dies here and now before going back to the future so he can eventually come back and do all those things, he would also wreck the timestream.
Basically, the kid version of Nathan is a frickin' idiot. Having said that, you make a good point about that probably being moot now.