Lets be honest here, people are still using Exchange 2003. Unless your business requires bleeding edge Exchange updates you can usually sit on the same version of Exchange for 7-10 years. Dont get me wrong I love new kit but spending more for the sake of it is exactly what Microsoft what you to do. I agree with the redundancy but space is relatively cheap and again I'm not talking about 10 users (Office 365 all the way) I'm talking about businesses with 150+. They would usually have inhouse servers and IT inhouse or outsourced looking after the IT anyway for their other needs unless completely cloud but thats rare at this stage. So adding Exchange tasks to that is not a biggy if Exchange is configured correctly to begin with. Office 365 is just Exchange online so how you setup users etc.. and the work involved is the same. And yes I'm not talking about backend database management/backups just purely from the administration point of view. I love Office 365 and refer it to most of my clients but not all needs are the same.
You are mixing and matching lots of different things.
- Not updating your systems has nothing to do with giving Microsoft money. Most places running Enterprise software are paying some sort of maintenance, which entitles them to free upgrades.
- Enterprise storage is incredibly expensive.
- Having in-house servers and in-house IT staff has very little to do with running Exchange. Messaging is a specialist subject, which is super high visibility (everyone from the CEO down are affected by any issues) and requires a lot of ongoing support. In other words, looking after a messaging infrastructure consumes a disproportionately high amount of time. It's also harder to hire generalists that are also good at Exchange, or you have to hire a specialist who is not going to be any good at anything else.
- "As long as Exchange is setup properly to begin with", but that is a huge assumption. Who exactly in your generalist staff is going to do that? Anyone can mount an ISO and click next next next. Setting up Exchange properly with all the necessary moving parts is a complex project that requires knowledge and experience.
- 150 users @ £7 a month is £12,600 a year. That's maybe a quarter of a half-decent IT person's salary. How are you going to pay for: software licenses and maintenance, hardware purchases/depreciation and support, storage purchases/depreciation and support, let alone hosting (rack space, electricity and cooling), staff salaries, training for your staff. I haven't mentioned compliance, archiving, backups, reporting... Oh, and don't forget for £7 a month you get full desktop Office suite included. I'm not saying it can't be done, but it's probably going to be pretty bare bones.
Bottom line, organisations with hundreds of thousands of employees are deploying Office 365. The numbers obviously work quite well.