Yeah, once you get that deeply entrenched in Oracle, it's almost impossible to get away, and after that experience I vowed never to work at another Oracle shop.
I wasn't directly involved in but had a good view of our university's finance modernisation woes: http://news.bbc.co.uk/1/hi/education/1634558.stm https://www.admin.cam.ac.uk/reporter/2001-02/weekly/5861/1.h... - although in fairness the inflexibility and disorganisation were existing features of the institution, and Oracle merely exacerbated them.
Ideally you'd have some kind of plan though from the start, for which other cloud provider you would use and how the services would map, in case using AWS becomes untenable.