By the time that a collision is anything even close to possible in a few trillion trillion years' time (let alone the address space being "saturated"), I'm pretty confident the human race (if we still exist or can even be recognized as "human") will have moved on to something other than bitcoin.
Consider, for comparison, that a 15 digit credit card number "only" has 1015 possible combinations. No one is ever concerned about credit card collisions, even though the credit card number space is a minuscule fraction of the private key space. For reference, the different between the credit card space and the private key space is several orders of magnitude larger than the difference between a single atom and the number of atoms in the entire solar system.