I tend to accept that sport and sportsmen tend generally to improve over time, and this goes for cricket as much as any game. If you parachuted Denis Compton and Ray Lindwall in their prime into Test cricket in 2009 they might not find themselves all at sea but I think they would find the overall quality higher than they were accustomed to. New skills and techniques have developed, tactics are much more sophisticated, and fitness, preparation etc have all improved vastly.
We should also beware the rose-coloured spectacles. They tend to distort our vision too much to be reliable.
And very rarely do you find a cricket writer who considers that the stars of the present (whenever that present may be) are quite as bright as the stars of his own younger days. This is human nature, of course. The rotten spells where Marshall bowled at half pace and didn't move the ball at all have faded completely from the memory while the vicious late inswingers remain, and such memories are unassailable, not least because we have a strong emotional attachment to them.
One day when I'm at a very loose end I intend to prove, by reference to the rose-tinted writings of a succession of reputable cricket writers, that some nondescript nobody from the 1860s was better than the finest players in recent history (on the basis that player A was compared unfavourably to player B, who another writer compared unfavourably to player C, and so on). I can't imagine it would be difficult.
Having said all that, I don't see why the greats of the past need to be done down in order to build up the greats of the present.
And don't forget that we would not have a Ponting if we had not had a Grace. Modern players are standing on the shoulders of giants.
Personally, I simply wouldn't put Grace and Tendulkar on the same ranked list of batsmen. They're playing a completely different game. There cannot be any basis for comparison other than the absolutely subjective, and one must throw stats and all quatitative evaluations out of the window.
You can compare them to their contemporaries. And that's the criterion which shows Grace to be as outstanding a player as was Bradman - arguably even more so.