There's no build-up. All the build-up was gone as soon as Zoro trained under Mihawk. How is Zoro vs. Mihawk, in any way, going to be this huge climactic battle at the end of the series when we know the Straw Hats are going to fight Blackbeard, the Admirals, and the WG illuminati? There's literally no stakes to the fight even if Mihawk gets wanked to oblivion.
I don't think Shiryu is going to kill Mihawk, but I sincerely doubt at this point Mihawk is going to be Zoro's final battle (or even a battle).