I think that such a scenario would have been disastrous for the Free People. A prolonged battle for Isengard would have cost dearly to Rohan in terms of manpower, which was already lacking on the Pelennor Fields in the cannon story. It would also have meant a delay for Aragorn's departure towards the Southern Fiefs... And we all know how that would have ended for an isolated Minas Tirith.
I think that such a scenario would have been disastrous for the Free People. A prolonged battle for Isengard would have cost dearly to Rohan in terms of manpower, which was already lacking on the Pelennor Fields in the cannon story. It would also have meant a delay for Aragorn's departure towards the Southern Fiefs... And we all know how that would have ended for an isolated Minas Tirith.