in answer to your first question - Are the 2 exclusive? Are there many school out there who have moved to a cloud based MIS but still running FMS? – the answer is that you don’t have to run them both and yes, there are many schools out there who continue with FMS but have moved away from SIMS to a cloud MIS. Recently I have worked with an eight-school federation who use FMS and Arbor MIS and are happy with the set up (more happy than before).
In answer to your next set of questions, it is not difficult for LA schools to move away from FMS and continue to meet their own, as well as the LA’s, requirements. Most recently, London Borough of Hackney LA helped its schools purchase cloud MIS and an alternative finance system to FMS – see this link - https://www.find-tender.service.gov.uk/Notice/028345-2021
And you’re right to say LA’s can’t dictate what software schools run!