Pray tell, who else is left Der Alter??
Obviously, the saints in the New Jerusalem have already WASHED their robes and made them clean. So who can it possibly be who are WASHING their robes? Must be those darn aliens, ETs huh? Since the saved already are wearing laundered linen, who can it be who is still wearing sooty clothing Der Alter? Fact is there is only one choice - the unsaved. And just where might they be? Oh yeah; those dogs, sorcerers, fornicators, etc. outside the City in the lake of fire, in v.15. Any other suggestions? Inquiring minds want to know.
Moreover, you overlook the fact that v.11 comes before v.12. Talk about cherry-picking without paying attention to context. V.11 states he/him, the unjust are still filthy. V.12 however states that Jesus is COMING [present/indicative] soon with His reward according to each man's work. Thus there is still time for those in the LOF to wash their robes. The invitation to come to Christ still exists. V.17 confirms this as practically the whole verse is rendered in the present tense. The ones hearing, thirsting, desiring are able to come to Christ and take of the water of life freely.