Not quite, because the punishment would correlate with the impact of their rule breaking.
If the punishment had to take into account or estimate the impact of the cheating, how could you accurately determine this?
If Celtic had lost on penalties, could the impact of that ineligible player have been the difference between winning and losing? You'll open yourself up to countless appeals from clubs and endless debates as to precisely what impact a certain rule violation could have had. It's a no-win situation.
It's easier to disqualify the offender as they've done in this case. The clubs all know the rules, and it's up to them to make sure they abide by them.
How could Legia make such a mistake? Surely given the amount of money involved, they have people checking details like this. It's really incompetence on their part, sadly.