The below offside calls were made by the linesmen. Seem pretty marginal to me. So marginal calls
will still be made, just as they were before VAR ever came in and just as they will be until the offside law changes. They'll just be made with less accuracy if left to linesmen.
And as for VAR judging based on the naked eye, that can be hugely misleading depending on the camera angle. The below onside decision wasn't even particularly marginal, yet how many people would have got it right based on the naked eye? Posters on here certainly thought it was an obvious offside.
Plus we already know exactly how your suggestion would play out, because they tried it in other leagues. It resulted in incidents like the below, where offside was given by both the linesman and the var's naked eye, even though he was actually onside.
You'll note there's still a line in that picture. That's because broadcasters keep applying their own lines to such calls, gleefully highlighting each and every mistake made. Which then results in fans complaining about bias, corrupt refs, useless VAR and the league stupidly not letting VAR use lines like every other major league does. Which inevitably resulted in them adopting the same system the PL are using.