The problem is that the Cavendish is sterile and only reproduces from cuttings. The seeds, those little dots inside, do not develop at all. So every Cavendish banana plant in the world is, in one sense, the same plant: genetically identical and cannot be bred from.
So you have to go back to another variety and breed in any of the qualities of virus resistance, productivity, size, flavour, edibility etc. which it does not currently have. Which is not impossible, but no such varieties have been produced in the last thirty years or so, so an estimate of decades dies not strike me as unreasonable.