I think that some proper research into megabeasts would be a good idea. In my opinion, megabeasts need a bit of an overhaul. I personally believe that the megabeasts should be something to be really feared, rather than jsut being a statistic in the legends for you to skim over.
Regarding titans and dragons, I've yet to see a single titan outlast dragons, in fact, as I said earlier, all my world gens have seen dragons live far longer, but it does seems that titans have a higher kill to lifetime ratio, perhaps this contributes to their earlier deaths?
As far as collosi go, I just genned a new world and found that there were a total of 5 collosi, 4 hydras. When i looked at the legends, collosi deaths were in the years 2, 3, 8, 11 and 27 which was amazingly low, and only the last two actually got any kills, but I guess there are some variations which cause my results to differ from yours.
Correct me if I have gotten this wrong but here are my general observations:
Dragons and titans live the longest, whiel dragons tend to outlive titans, the titans normally kill more in their lifespan.
Hydras and collosi live comparitively shorter lives, and it seems to be variable how long each live, sometimes the collosi will die out early, while hydras live for a long time, sometimes the other way around.
The semimegabeasts are generally dominated by giants due to their CLUSTER habits, while I havent actually studied the legends for the SMBs, a skim over the few worlds I have and their world gen population distribution files seems to suggest that there are a greater number of giants than other SMBs, but as I said, I havent studied them.