No the offense and defense doesn't have an equal chance to score, I agree with you there. However, when you consider that the defense can score or they can stop the offense and get the ball for their team, it becomes much closer to equal. Hence, you see stats along the lines of what I posted - 43.4% of teams that lost the coin toss win the game.
So, how unfair or uneven is it when you have a 43.4% chance of winning? Yeah, it's a bit of a disadvantage. I agree and the historical numbers agree. However, do those odds necessitate changing the rules? I'd say no.
The biggest problem I have here is this gets wrapped up into the whole notion that everything in our life has to be fair or even. Life isn't fair. You had a chance to win in regulation. You have a chance to win in OT. Is it always fair/even - no. Home teams, teams have better players, some teams cheat, some teams pay off refs, some teams have better coaches - there's all sorts of advantages for teams during a game. It's never even.