He won't come back, he's a devout guy and a big part of his culture is respecting your elders. The Ferguson boat has sailed and Justin gave him the sort of beating that changes you permanently - he would have got destroyed too IMO, so between his mum's wishes and not wanting to fight someone he's already beaten again... that's that.
The plan was always for Islam to take the mantle once Khabib retired as the lightweight guy so we're just going to see that now. Hopefully he can carry on the maulings for our entertainment.
Also haven't seen this posted but Chimaev is supposed to be fighting Edwards now, a massive jump up as obviously Dana is trying to get him into the upper echelons of welterweight ASAP.