The "default" would be to play the full durations indicated.
That should not be confused with thinking there is no room for expressive interpretation. It is also a sort of "default" a performer is expected to give some expressive interpretation. Otherwise, performance is just a robotic activity, like a player piano roll, and the artist performer is irrelevant. How to interpret is the art of performance.
A lot depends on the era of the music. Compared with Renaissance to early Classical, the Romantic and Modern eras put more detailed expression marks/performance indicators into scores.
So, why do we need slurs to indicate legato?
Additionally, slurs indicate phrases. So there is more to it that just legato articulation.