The two problems with that idea is that it wouldn't in any way eliminate mm offside calls and it would have massive knock-on effects in other areas as it's an enormous advantage to hand to attackers.
For example, in the case of a Man City free kick Sterling would be onside in this position:
Defences simply can not allow attackers to have that advantage, which means they would instead have to pack the box with defenders in such scenarios. Similarly, the high-line so many teams play with in open play would no longer be viable. So while it seems like it would give advantage to the attacker and add more goals, the knock-on effect would probably be much more defensive tactics within the game, which isn't something you want law changes to encourage.