1. the private use area
2. the general "CJK" area
The second of which has a truly mind-boggling number of characters, including every possible composite Hangul glyph used in modern Korean, despite them being constructable from the basic Hangul codepoints.Emojis and other symbols which aren't used for language appear relatively rarely. Certainly there is no reason to believe that UCS-2 would be sufficient for writing if they were removed. The number of scripts included in Unicode would exhaust even the private use area, and UTF-16 would have been invented regardless.
Also true of most Chinese characters, but the proposal to encode them component-wise was a no-go (for adoption in China IRRC) and separate character encodings was went with in the end. I never managed to dig up the reasons behind it.
Unicode strives for the round-trip compatibility with source character sets, and in this case KS X 1001 (KS C 5601 at that time) is a main culprit: it had 2,350 (out of 11,172) common syllables precomposed. But it happens that Korea had supplementary character sets beyond KS X 1001, which were subsequently added to Unicode 1.1 (up to some 6,000 characters), before it was decided that having an algorithmically derived section of all 11,172 syllables is better. This whole situation is now known as the "Hangul mess".
Nothing trivial annoys me more than people writing in "I luv u" shorthand, so if an emoji can a more emotional message in less characters I'm all for it. Even if it's a thinly veiled sexual euphemism.
Emojis in official corporate communication can burn - I got one recently when applying for a relatively serious job: sends a strange message, that and it reminds me that I want to save the emojis for my friends and family (sadly not the people we most of spend our time with in)
Also, fun fact that I just learned: CoreText synchronously calls through to a font registry in fontd using XPC to draw text on the main thread in your app.
In my opinion Unicode went from a slow-moving standard that carefully absorbed the world's languages, to a pop culture product that serves to make old software obsolete even faster.