Abaye is saying merely that according to the logic being presented by Rebbi Zeira -- that we push off getting married due to this possibility which is a need for after Shabbos -- we similarly should make sure that Yom Kippur does not fall on Monday but rather on Tuesday. (The Gemara promptly differentiates between this case and Rebbi Zeira's case.)

If you are bothered because this is right before Yom Kippur and thus how would anyone come to transgress the prohibition against doing Melachah on Shabbos, the answer is that Gezeiros are generally made even when people are in a holy time or situation. For example, in his famous comment, the Moshav Zekenim writes that the Torah prohibited a Kohen Gadol from marrying a widow because he might Daven for a married woman's husband to die when he is in the Kodesh ha'Kodashim on Yom Kippur, in order that he be able to marry her. It certainly is possible (within the realm of the Gezeiros of Chazal) that a person will have a very busy schedule and be extremely preoccupied and end up slaughtering on Shabbos.

All the best,

Yaakov Montrose