Let's open the playbook a bit.
Disney vendors likely provided life cycle expectancy, based on empirical evidence, and had forewarning of impending issues by exceeding performance limits. So the issues experienced over the past, let's call it 2 years for arguments sake, could/should have been fully expected. If the vendor did not provide failure metrics, then I would have questioned that vendor's capabilities (but that's just me). If they supplied a 20-year operational limit, and you, the customer, decided to exceed those limits than you assume full liability and all costs associated with maintaining the design's original intent.
Agree with your statement about the theater money not being "directly" tied to the monorails, but I suspect each park is given a projected budget (now whether MK/EPCOT cost share the monorail is another discussion).