“best thing since X”

Solution 1:

Typically the writer of such a phrase expects the reader to infer that Y (the earlier event) was bigger, deadlier, or higher than X (the recent event); and typically that is what readers do. It is true that in point of fact such phrases don't actually say, and don't logically imply, that conclusion. But in ordinary conversation the implication is taken for granted.

Solution 2:

Taken at face value, it does imply that Y was not surpassed, but its use is often figurative, such as in, "This is the greatest thing since sliced bread," and not meant to be taken literally.