I want to say that it is because we have a Halachah (see Rema, Orach Chayim, end of 138) that you should always finish off with a good thing. That is why the last two words of the chapter are "and it is to be preserved." A few lines before we had a Derashah "to guard and not to destroy." It would not have been so good to finish off the chapter with the word "to destroy." It is much better to make a Siyum with "to preserve."

On Wednesday morning the fistborn will make a Siyum, bs'd, and we daven that this will mark the end of our troubles and it will be a Shemirah for us and the whole world!

Pesach Kasher v'Same'ach,

Dovid Bloom