If the thicker lines overlap (which they would when it is a very tight call) it would not be called offside, i.e. the benefit of the doubt is given to the attacker.
I think that would require a law change, so would be much slower to implement... but I agree with you, and it will quite likely be the next change they make.
https://www.bbc.co.uk/sport/football/57435400
VAR: Premier League to use thicker lines when assessing offside calls next season
A good move I think - gives a margin for error on the tight calls (armpits / toenails / which freeze frame is used etc...).
If it is given as offside with the...