The vowel you’re talking about is “ə”, also known as “schwa”. It’s probably the most common vowel sound in all varieties of English - in defiance of spelling - but it occurs in lots of languages. (German often uses it as the sound for final e, and French still retains it in some words and dialects for the same thing too.)
As to how common it is in English:
“Thə vowəl yo~ə’re talking əbout is “ə” also known as “schwa”. It’s probəbly thə most commən vowəl sound in all variəties of English.”
In some varieties ə moves closer to ʌ, a similar sound that’s a little further back in the mouth. Links go to Wikipedia where there are sound files to listen to.
Illiteracy possibly. Almost certainly more people will be less able to read older texts.
If that’s going to happen anyway, they could do worse than adopt Pinyin or some variant of it. Or they might prefer something like Bopomofo if they don’t want to use Western characters for whatever reason.
Either way, any decision like that is likely to be 20-30 years away at minimum, and that’s assuming literacy rates start going down, which they might not. I doubt Xi gives it any thought at all.