Let's try a different tack.

Say that you put out a call for conference papers, but due to a bug in your mailing list software, only every other email is sent - 50% of potential participants are excluded. What do you think will happen to the quality of your conference?

Now let's say you find the bug after you've decided on your talk schedule. Should you should fix the bug and include the 50% of papers that you've missed? Isn't that unfair to the people who've already been selected?