While we all realize the pointlessness of speculating, I think it's pretty safe to say that losing a player of such quality, on balance, generally hurts you a lot more than it helps, especially considering the drop off - maybe they still would have lost - we'll never know - if Zaheer had been fully fit all five days. But I'd put my money that it would at least have been closer (nothing is a guarantee but the odds would be in favor of that).
No one is saying that England don't deserve to win, or that they wouldn't deserve the #1 spot if they win by a two test margin. Different teams are capable of coping better or worse with different losses. I imagine loss of Swann would hurt England the most (in terms of bowling), whereas Zaheer is that person for India. However, even with that England is better equipped to lose any one bowler than India is because aside from Swann, the drop off from #1 to the backup isn't as steep as it is for India. That's a strength England has.
Obviously, as a fan, I would love to see Sehwag, Zaheer, etc all fully fit and I think we'd have a cracker of a series but I don't think anyone is using it as somehow saying that it doesn't 'really' mean India didn't deserve to lose, etc. Injuries are part of the game. India drew epically in 2004 in Australia because their bowlers were hurt, and England likely wouldn't have won in 2005 if McGrath hadn't gotten hurt but that's part of the game - Australia couldn't cope without their bowlers, but that didn't make any of those victories and losses less legitimate.
England won by 200 runs despite batting on the first day. Well done and fair play to them. I think every Indian fan has congratulated England on their victory so I'm not really sure why there's been such a hatred on Indian fans lately (hating the Indian team I can understand, people love to hate sportstars or teams for any number of reasons - and in fact I think it's part of what makes sports so awesome).