Well, I think we can be sure, in light of a whole lot of other things, that CC is not always in accordance with itself in terms of temporal events and such. Also, as I have pointed out in regards to the Frozen Flame in another topic, it seems to, at times, contradict what is shown in CT, and one can only decide whether to trust the older or newer game as to the matter of the truth.
Now, I would like to believe that Crono lived. If nothing else, it would make for a nice cameo should Chrono Break come out. Upon reading these replies, I find myself at somewhat of a variance in regards to his living or dying, however. Both sides bring up valid points, and I am unsure which is the correct one, if there even is one truth in this matter.
It is true that should, at some point, Guardia fall to Porre, Lucca would see this in her summoning of Doan at the end of Chrono Trigger. But I am not entirely sure whether Doan's existance proves Guardia's. I cannot remember, but does it ever say that he is a descendant of Guardia? Though the land's name may remain, the people may long ago have been assimilated into Zenan's main culture, even as Brits don't exist in Britain anymore as they did 1500 years ago. As for the other hints towards Crono being alive...I should think that Lucca's referal to someone coming to kidnap her friends is a pretty strong indicator of life, unless 1)she, knowing of the intricacies of time is speaking in some strange extra temporal sense (unlikely), 2.) the letter was written at some earlier time, before the fall of Guardia, 3.) the designers put little thought into the tenses in which they wrote/translated. It is the third that really makes me wary of this theory. And now we come to Crono's death.
While Crono can certainly take care of himself well enough, we must note that Guardia did indeed fall. This means that, for better or worse, Crono failed as prince in some regards, and let his land be overrun. If he lived through this calamity, then he is without a doubt an outlaw and guerrilla warrior. No man can wage a fifteen year war without it becoming a hit and run type. But if we look at such a thing realistically, he would likely have died. First of all, unless he gained some wisdom since his younger years, he would stupidly stand by the castle until it fell. Secondly, even if he did survive there is a great chance that through some trap he was killed in the intervening years. Maybe not, he is a very lucky warrior, after all, but this is a very likely thing. It is simply theory and interpolation of events, however. What we do know is that the ghosts of Lucca/Marle/Crono profess themselves to be dead. The only event that changed between the end of CT and CC concerned El Nido, not Guardia, and so if they are dead in one dimension, they are certainly dead in the other. This seems near iron-clad, but for the fact that they are ghosts. They may not be speaking in the strictest sense that we know. They may simply be a shadow of what may have been, some other future of the three, that no longer exists; they never say that they are dead, merely that they no longer exist in this timeline.
All these are certainly speculations, and really don't resolve the issue, I know. I've just summarised several of the things that have been said and that are already known. My own theory, stepping back and trying to take the view of the designers and what the intent was, is this: they are dead. It makes sense from the designer view; make a game in which the old heroes are all dead and gone, and leave the heroics for the new generation. The old wizard who helped the last company now joins the new; he and his sister are the connecting pieces between the two tales (not in the final game, surely, but I have heard that Guile being Magus was at some point intended). The torch has been passed on, in a sense. Though the letter seems somewhat ambiguous, a lot of writing can seem so if carefully scrutinized. I wager it was simply meant to be a letter from beyond the grave, a last call from the old group to the new, reminding Kid of her task and, in the earliest drafts, revealing the mysterious magician to be their old nemisis turned ally Janus, who is named for the Roman God of Beginnings and Endings. I think that the intent in this letter was no more than this, and was not meant in any way to suggest the the three live any more. Likewise, though the ghost's words can be taken somewhat ambiguously, I don't think that ambiguity was the intent when their speeches were written. If the writers never had any intent to bring back the three, then to kill them off distantly makes sense: it's the old shock value/ nostalgic sadness trick to add emotion. They killed Lucca, why not Crono and Marle? This is my opinion on the matter, from the overall impression I am left with.
Now, all this isn't to say that SE, if they ever make a sequel, will not find some way to ressurrect Crono. Many loopholes exist, to be certain. Or they could just go fully against what was said in CC, as CC at times contradicts CT, and pull them back out of the blue; their deaths were not put into the forefront anyway, and would not constitute a major breach of continuity.
Hmmm...I wonder if anyone will reply to this. The last few lengthy posts I have made seem to have been greeted with utter silence. Is it just an illusion, or is my posting a harbinger of the death of a thread?