Lots of people misusing a word doesn't make the misuse correct. The word has a number in it. Numbers dont change value. People say 'decimate' when they mean 'devastate'. If the larger part is destroyed, use the latter.
Many words are commonly misused, 'chronic' for instance, many use it as a substitute for 'accute' when they refer to an ailment or pain. chronic of course means re-occuring over time, or persistent.
'Ultimate' just means the latest in a series, it does not imply the object is the best. Does penultimate imply second best? Ultimatum means we are at the last option, not taking the best option.