Games have always been pretty linear, though. The ones that had a notable degree of "freedom" found that freedom in

a) Choosing which enemies to go and kill with your chosen color of pixel burst.

or

b) Choosing which set of text and vaguely representational spritework the game would expose to you.

Many "linear" games offer tactical rather than strategic freedom. There is a sort of conservation of experiential depth, limited by the players' ability/inclination to absorb new interaction concepts, and the availability of developer resources to build them.