I don’t recall ever agreeing with Avi K before, but his point here seems valid. If you want to speak Yiddish because of its long-time use and all that, then that’s one thing. But there are real problems with Yiddish-izing lashon haKodesh, especially how yiddish tends to be largely mil’eil while Lashon haKodesh has many words that are milra.
For example, using the typical mil’eil pronunciation of Yiddish, people mispronounce words like the first word in shema after the two initial declarations, by stating viaHAVta eis instead of ViAhavTA eis.
How people speak Yiddish is not a real concern. Yes, languages can and do evolve; that’s fine. But not lashon haKodesh. G-d made that the way He wanted it. If you improperly pronounce Lashon HaKodesh when davening and laining – if that’s improperly influenced by Yiddish (or for any other reason/culture/whatever) – then that certainly is a concern.