It would be insane. Because, a legendary is contributing to the forum far longer than the merit system exists. It is very much possible that a hell lot of posts created by him/her in past were worth of merit, but it were not awarded any. Now, if only the earned merit is considered, it'll be highly unjust to the old posters.

For example, there was no merit system in 2014. But, if this thread was created today, it probably would have received some merit. This is how many of our old threads are merit less, because they were created way before the merit system.