Finally some sort of comparisons ARE possible.
There were people who watched, played with or against Grace, Ranji, Trumper and Hobbs who have written in detail about these champions and even compared them There are more than one such account available to have a reasonable idea.
Then there are those who saw Hobbs, Hammond, Bradman, and George Headley play. The number of books and accounts are numerous.
Those who saw Bradman, Hutton, Compton, May, Harvey, Weekes, Walcott, and Worrell are too many for you to even read all the accounts.
Those who saw and wrote of the three W's, Sobers, Hanif, Barrington, right upto Gavaskar are even more.
Those who saw everyone from Tendulkar to the present day cricketers are still hail and hearty and younger than yours truly
The point is, that if one wants, there is a thread of continuity running through cricket's chronicles which can be followed. So when people like me or others form some sort of an opinion about one player being the equal of or distinctly better than another, often, a the back of it, is an opinion which has been formed over a period of time during which one has absorbed a lot of different opinions about more than one generation of cricketers at a time.
Of course it would be interesting if someone was to put down on paper a kind of "140 year poll" of the top cricketers of all time based on this kind of linking of threads, it may serve to prove some quaint idea. But finally the exercise is going to become about us and our own biases or prejudices or our need to have the last word. I think it can be done but whether it will serve any purpose, I am not so sure.
By the way, whenever, I have come across people who have very low opinion of the older cricketers, I have invariably found that a couple of questions will reveal that they have hardly any personal reading on the subject. Similarly those who do not denigrate old cricketers will by and large have a good amount of cricket reading behind them. While this doesn't prove anything regarding which might be right, it does show that many of us (right or wrong) hold our opinions without even making an effort to verify them.
That is a tragedy particularly since most of them are really lovers of the game. Since pictures and videos of older cricketers are rare or not available, written accounts are our only source. For those who have time but no inclination to read up, I can only say, "you may be right in what you are saying but you really dont know that is the case."