A full implementation/rewrite of something like word, which is pretty complex and the quantity of edge cases is insane, in a relatively short amount of time, months, is very hard.

If they succeed with AI it will be huge: it's basically not possible to do it today with humans, no matter how many people you put on the job.

I would agree a full rewrite of word which is very complex and take quite a bit of time.

It would be impressive of AI to make a 80% rewrite, though I fully doubt it would cover the 100% rewrite (disregarding long term support as well).

Why do you think it’s impossible for humans to rewrite word, especially even humans made it in the first place?