There are times when 400mm is not enough.
And like buses, there aren't any 600mm lenses second hand for months.
Then three come along at once !
I'm looking and there are a number of 300, 400, and 600mm lenses that look legit.
I bid on a 300 last year but it was a scam on a stolen ebay ID - the auction was pulled by ebay within 5 days.
So I might end up with a 300 f2.8, or a 400mm f2.8, or a 600mm f4.