It's true that the 49ers, Cowboys and Steelers dynasties all established country wide fan bases but it isn't a guarantee. Look at all the success the Patriots have had and they don't have the same kind of fan base. If they Rams ever become a dynasty of any particular decade, I think times have changed too much for that alone to generate a fan base like it used to. The dynasties of the 70's, 80's and 90's were different than anything post 2000 IMO.