They're still going to have to draw lines for the daylight rule, deciding whether there's any part of the attacker that's level with defender is just the same predicament they find themselves in now where they need to draw accurate lines for both the defender's and attacker's "active" body part that will determine the offside, and ensure they've chosen the right frame where the ball is kicked.
If you're suggesting "if it's too close to call, it's onside" then that's something completely different, that could be implemented under the current rules.
I'm also not sure what the effects of the daylight rule would be on the game, defenders can never truely be goal side when they're defending unless they drop ridiculously deep.
The issue with this is it opens up for further levels of inconsistency and accusations of bias. Who decides what sufficient levels of doubt is? If one week VAR reckons a player is maybe 10cm off but it's too close so call it onside, what happens the next week when they rule out a goal that's similarly close? The team who have their goal ruled out won't be happy and neither will the fans, and you can bet there will be cries of corruption from the fans. If 10cm is an acceptable leeway what about 11cm? 12cm?