Your villagers are smart enough to know that they don't need to research anymore after you have purchased all of the levels of Village Tech. There is really no need for scientists at that point in the game.
The population limit is 90 villagers in VV1, although
this thread discusses how it's possible to have more than 90 villagers.
There are several discussions about how to make villagers live longer, which you can find by searching (make sure to use the advanced search, so you can change the date range to 2 years). Here's two threads to get you started:
look here and
here.
