Also, very very skeptical that Seimens ever offered free monorails. Those monorails would be worth A LOT of money... way more than Disney would have sought to charge for a sponsorship renewal.
The following is completely anecdotal. That said:
A friend of mine was working with Siemens around that time. According to her, Siemens pulled their sponsorship (at least partially) because Disney declined a contract for new monorails.
I certainly have no proof or corroboration, but that seems a lot more realistic to me than Siemens pending over backward and offering free monorails just to keep their name on a ride. By the same token, I can't imagine Disney turning down such a one-sided offer. I dunno what it would cost to replace the monorails, but I'm sure it's far above the cost of sponsoring an attraction.