Hey now! No actual reading of the source material please. /s

It's more or less a matter of time until everything will be migrated to go. Since it has better performance and works good for them, there's no point not to do it.

Not necessarily. First easy reason for not migrating: the cost to do so outweighs the benefits. Why port working code to a new language when all you're doing is maintenance?

I don't know about you but its hard to sell upper managers on complete rewrites of things when the end result is: no real change but it might/should/could run faster. Unless performance is a concern to be addressed the risk of changing technology stacks doesn't seem a great idea.

You don't have to rewrite anything, the old language can still die. If you're writing all your new code in X and your codebase growth is accelerating, the existing code in Y will look less and less relevant over time.

Perhaps, but it doesn't sound at all that the case is all new stuff is go for them.