I think that the Lab could have some sort of bonus to the Residents that have more than 10, even 15 years of continuous Premium membership. Those would have direct access to "Super Premium" membership with bonus price or even a one time payment. But what the Lab really wants is to have more and more Premium members, not worrying too much about keeping the current ones.
And yes, I would spend that kind of money for a 1/4 of SIM (It's what I have for almost 10 years in Plusia, Mainland), if paid only 1 time. After seeing what is happening with Sansar, I fully believe that SL will be around for many more years.