I'm sorry if the issue was that important. I have been involved in a lot of contracting and have seen many requirements drawn so that only one product would fit the bill, even if the one defining requirement was a cupholder. For example, instead of stating the free/busy architecture must not cause performance problems, you state that you need a specific free/busy architecture and describe the one that's in the new Exchange.
Move to iCal and no more Jet database. As far as the architecture, I don't even know if the problem exists for iCal. Exchange's problem could have been another case of Microsoft engineering itself into a corner, not applicable to anyone else.
If that's all there was to it, it would be an easy decision. You keep talking as if Apple finally getting OS X right and being cheaper, for the moment at least, is all you should need to know to start tearing out your existing infrastructure. I've seen lots of companies come out with a product that's "the next big thing" that's going to unseat all their competition. So far they have a history of being able to build one good, enterprise class OS, and priced it cheaper than Microsoft. You ask too much on that alone.