But then that would incriminate them even more! right? because it's not just "We erased it to make room for new data" ... This is in the realm of obstruction of justice.
Yes and no. If the "standard procedure" when they normally wipe the data as part of routine is to overwrite (Which I'd probably hope so) then the fact that they did so a) Makes it impossible to recover, and b) Doesn't make the action any more incriminating.
This is entirely separate from questions of obstruction of justice.