How good would they have been?

We can only speculate but two players who died tragically young spring to mind:-

Klaus Junge--killed in the last days of World War II aged 21. At Prague 1942 came =1st with Alekhine who spoke very highly of him.

Gordon Thomas Crown--a great hope in British chess. He died during an operation for peritonitis aged 18 in November 1947. Won a strong Hastings Premier Reserves in 1946-47 and beat Alexander Kotov in the Anglo- Soviet match in Sept. 1947.

or how good would blackburne have been had he started playing chess at age 6. He began at 18 and still played the very top of the world.

Since Blackburne lived to be 83 I think he had the time to get to the peak of his ability. The point I'm making is that Junge and Crown didn't.