I think that thanks to the internet and pseudo hive minds of Gen Z (or millennials?) and younger, human society will remain mostly stable for the foreseeable future. There will be civil unrest and power transitions, but our technological civilization is on the brink of becoming immortal within the bounds of time.

So maybe the US government will collapse (hopefully), but it will be replaced by something quite similar. Just as it was a replacement for older forms of governance, derived from the old Roman and Greek republics.

It already has. You would not understand a man from 1500 England, and you might barely understand some AAVE, Jamaican Patois, or Scots speakers of today

I think people will read the English literature of the 1900s forever. And I see English is gaining adoption.

I assume eventually, there will be a spelling and alphabet reform

Like why does “spelling” have two Ls instead of more clearly representing the sounds by marking the E as a short vowel? In the Shavian alphabet, it would be “𐑕𐑐𐑧𐑤𐑦𐑙", two less letters and completely clear in pronunciation for foreign learners.

I don’t think they will use kanji specifically.

I do agree simplification is inevitable. But what you are sensing is the constructed language that will replace natural language. It will use words and symbols derived from English, Latin, Mandarin, Arabic, and others. Probably some tyrant empire in the future will enforce it