Trunks is Half Saiyan though, and for whatever nonsense reason they get stronger faster than full blooded Saiyans. I honestly think he would have kept training with Vegeta ferocity because he knew that he was the only person who could protect the earth in the event that something happened and would have achieved Ssj2 (he saw it and knew what he did wrong so could easily get it right) and eventually three in a moment of desperation against fat Buu.
No way the story could work if Super Buu or Kid Buu came about though, Trunks would die.
not trying to nit pick but ive also operated the assumption that being half sayain gives you an advantage but is that actually stated anywhere officially? because to me it seems that whether or not the person is full or half, given the opportunity to to train or get angered everyone has the ability to get stronger regardless of how much saiyan is in their blood. and as for trunks i really dont know there is only so far anyone in dragon ball z has gotten with out training under someone or fusion.
Nah man its not even nitpicking, I'I can't even remember if it was officially stated anywhere myself either but somehow we've all just come to assume that. I don't think it's just totally made up though.
@KingdomFlatbush come clarify this.
That's also a fair point, Goku only achieved ssj3 when he was dead and could train for longer, who knows how far into that seven years he was dead did it take to even reach that level.
Unless we're going to cheat and say that because it's a cartoon and re hero has to win somehow(basically my logic) then there's no concrete reason as to why Trunks could even reach Ssj3 or beat Buu..