I think loli-Vinci is there to interject some unbridled optimism into the story. Like, GO made a deliberate choice to not make the lostbelts the fate zero of the grand order by having everything be relentlessly negative and grim, so da vinci is there specifically to make sure there is someone both intelligent and hopeful to give the story some much needed brightness.
holmes does it a bit but he is a bit more devil may care “eh, hope we don’t die!” Whereas davinci is someone who can say “we got this” and come across as believable. Especially since mash is having a moral quandary and is more conflicted.