There have been many 4chan spin-offs. 8chan is not the first or the last. If a google trends comparison means anything, it doesn't look like there's any fear about 4chan ever being surpassed soon.

>he responded to repeated questions about 8chan by talking about just about every other 'chan'-style site on the internet, making the omission very obvious, and indicating it was something he took personally.

Perhaps he didn't want to feed the trolls in his farewell.