Wait a minute, wait a minute. Everybody dies some time. Assuming that you mean 1020 AD. Ayla, Magus and Frog have to be dead that time, unless they time-travelled. Robo is a part of Fate. So, the problem is Crono, Marle and Lucca. The game doesn't say that they are dead. Especially after seeing Lucca's letter, although she might have written it years before, we have a clue about her. Crono and Marle probably lived in Guardia until Porre attacked. They must have died there, since I'm sure that after defeating Lavos they wouldn't flee in a war. However, this isn't certain either. But even if all the characters were alive, it wouldn't play a great role. The characters of CT have already done what they had to do. Those characters were complete, new ones were needed for CC and new will be needed, should SE releases CB.