I wouldn't be retiring Walker just yet, he got badly KO'd and should take at least 6 months break, but he's only 32.
It wasn't his best fight, I'm not sure why he/his team decided not to utilise jabs at all, given his reach advantage, I think that would've been a good idea.
I think he's just not good enough to be matched up with the top fighters, but could act as a "gatekeeper", someone who prospective fighters have to defeat to be considered worthy to be in the top.
I think matching him up against Dominic Reyes next would be a good idea.