Hebrew is not based on Yiddish, lol; only Ashkenazi Hebrew pronunciation was influenced by Yiddish. Modern Israeli Hebrew uses primarily Sephardi pronunciation, and Ashkenazi is mocked (i.e. Shabbat is Sephardi, Shabbos is Ashkenazi; modern Israeli Hebrew uses Shabbat). I grew up around Ashkenazi pronunciation in America, and had to unlearn it when I spent time in Israel. Nonetheless, Yemenite, Sephardi, and Ashkenazi Hebrew — the three major extant pronunciations, only one of which was ever influenced by Yiddish (Ashkenazi) — are all extremely similar and mutually intelligible, and thus all of them are extremely well mapped to the alphabet. Yemenite is most likely closest to the original spoken language, specifically the ע, but there are very few differences. And a modern Hebrew speaker can easily understand Biblical Hebrew — they're closer than even Modern English and Shakespearean.
Also, not all colloquial dialects are mutually intelligible. Different Chinese dialects are still often referred to as "dialects," despite not being mutually intelligible (e.g. Cantonese vs Mandarin). While that's typically mostly the case for Western languages, there's a spectrum even there.
> And a modern Hebrew speaker can easily understand Biblical Hebrew — they're closer than even Modern English and Shakespearean.
Of course, because modern Hebrew was constructed based on (the modern understanding of) Biblical Hebrew around the 1920s or slightly earlier, whereas Modern English naturally evolved for ~400 years from Shakespearean English and other forms of English.
That’s simply incorrect. Most of the innovations in Modern Hebrew (relative to Biblical Hebrew) came in the Mishnaic period, early CE. Hebrew continued to be used as a liturgical language, and occasionally a business language, both in its Biblical and Mishnaic forms, until the 1880s (not 1920s), when the Zionist movement brought it back into use for casual speech. The Hebrew used in the Mishnah is quite close to the modern written language, though it lacks modern words and some very recent innovations like topic-first sentences.