a few points
1) Joseph’s first topic was most on point, vesu lo midi
2) “Our common language is Hebrew. Why arent’t these rabbis speaking Hebrew?”
Why is our common language Hebrew? and since when? I don’t speak Hebrew
3) The OP asked “I assume nearly all of the attendees who do speak Yiddish also understand English very well.” you assume incorrectly. As you correctly note “t is meant to be a unifying event to inspire and celebrate”
Thus in order to INCLUDE (your word) Yiddish is added. There are not as many only Hebrew speakers at the US Siyum Hashas, if there where of course there would be languages in Hebrew. I’ll bet at Siyum Hashas Languages in south America there are speeches in Spanish and Hebrew, since those or the languages spoke n by attendees there.
4) “Would you invite someone to your house for a meal and then speak to everyone at the table in a foreign language?”
Yes! I dont skip benching just because someone at the table doesn’t understand the words. I explain what we will do, give him an English bencher, and go on speaking in Lashon Kodesh. Is this really a strange idea?
d o yo insist on skipping benching (or benching in English) just because someone at the table doesn’t fully understand?
5) At Harvard’s graduation (lehavdil) a speech is given in Latin, there are less Latin speakers who attend the Harvard graduation than Yiddish speakers at the Siyum. So why do they do that ? That I can tell you in one word: Tradition!