At 25, has the chance to live longer then just till the 60s. His own father, Owen Tudor, lived until 4 February 1461 reaching the age of 68. While his own son, Henry VII Tudor, lived to 52.
So lets say he is at least able to live until the age of 60 (mid way between 52 and 68

) this takes our Lancaster to 1491.
With his half brother dying in 21 May 1471, Edmund would be in a strong position to place his 14 year old son, Henry Tudor, 2nd Earl of Richmond onto the throne, acting as regent.
(Henry's claim is stronger then Edmund's due to his mother, Lady Margaret Beaufort, being a great-granddaughter of John of Gaunt, Duke of Lancaster, fourth son of Edward III)
With Henry being able to claim the throne over Yorkish claimant Edward "IV" Duke of York in 1471, the marriage to Edward's daughter, Elizabeth of York, would still be an important move to gain him Yorkist support away from the controversial Edward.
Along with Henry, it is also likely that Edmund and Margaret, would go onto have more children.
Margaret was only 13-year-old, when Edmund left her a pregnant widow.
Tudor Family Tree:
Edmund and Margaret Tudor
- Henry (28/1/1457 – 21/4/1509) M. Elizabeth of York
___________- Arthur Tudor, Prince of Wales (1475)
___________- Margaret Tudor (1479)
___________- Henry Tudor (1483)
___________- Margaret Tudor (1486)
___________- Edward Tudor (1490)
___________- Jasper Tudor (1492)
___________- Mary Tudor (18 March 1496)
- Edmund (1460)
- Margaret (1463)
- Owen (1467)
- David (1470)
- Katherine (1475)