Defending Minas Tirith a strategic error?

[Tuor of Gondolin]

Many good points above, and frankly, I would incline towards defending Minas Tirith, just not to the end, but at a given point pulling back through the mountains and back to Rohan. But for arguments sake:

Some comments:
1) The Witch King would be destroyed anyway when the Ring was destroyed.
2) You could envision a scenario where Eowyn killed the Witch King in the East March , edoras, etc.
3) Aragorn still might have a strong claim on the throne if he brought the Army of the Dead to free South Gondor and then harassed part of Sauron's army from South Gondor (and remember, his forces could always retreat through the Anfalas. Imagine Imrahil and Faramir leading M.T. remnants through the White Mountain passes to link up with Aragorn.
4) Rohan's cavalry was probably superior to Sauron's, and they would have had some 10,000 available in Rohan. On their home ground, and with strongholds in Eoras, Helm's Deep, Dunharrow, presumably Orthanc, etc., and with routes of retreat back through Enedwaith they would have been formidable. And battles in history have shown that forces fighting an invader tend to increase in efficacy. One example being the better performance of the Army of the Potomac against the Army of Northern Virginia at Gettysburg, where it was essentially on even numerical terms and arriving in a scattered manner.
5) The retreat would only have to be long enough to give the ringbearers time to succeed.
(And I suppose JRRT might still have made a scenario to save Frodo and the nassty, fat hobbit).
P.S. If destroying the Ring was all important, and the elves were leaving Middle-earth fairly soon (and can eventually be reborn in Aman) why didn't Elrond and Galadriel have at least part of their forces engage in fairly suicidal local offensives to further distract Sauron?

P.P.S. Anyone notice how "counterfactual histories" are rather popular in science fiction and general fiction, such as Newt Gingrich's Gettysburg-based American Civil War novels?