eyefortruth,
I think you have the Rambam’s shittah wrong. It has been a while since I looked into this, but if I remember correctly the Rambam holds that sometime after Moshiach will be techiyas hameisim where people will rise from the dead. That part is in line with most other Rishonim.
However the Rambam holds that the period of techiyas hameisim will eventually end and then Olam Haba begins where our neshamos will exist without bodies in purely spiritual form. The Ramban and other Rishonim hold that the period of techiyas hameisim is the final period (and therefore is what is meant by the phrase “Olam Haba”) and we will go on indefinitely in physical form.