I got to 900, and got a sentence or two, then was completely lost until around 1500.

This was fascinating, there was that one sentence around 900 but I'm fairly certain almost everyone who is a native English speaker will only pick up somewhere around 1400. Up until 1400-1500 it's mostly like listening to something completely different. I wonder if in another ~800 years people will barely be able to understand our language?

I think it's unlikely we'll see another major shift.

This one is the result of the Norman Invasion in 1066 - it just took a few hundred years to get to Shakespeare's time and modern English - which is also a result of English becoming the common language with French being the language of the gentry.

Unless there's another language invasion, it's not likely to change so much.

Just look at the difference from British to American English over a ~300 year period, and that's with both Jefferson and Webster intentionally trying to make American English uniquely different from British English.

900 I picked up a few words but then very little til 1400