I think everyone acknowledges that WinPhone is playing a come-from-behind game. As such, it is entirely reasonable to expect a fast pace of development from Microsoft (and Nokia), as missing features get filled in and wrinkles get ironed out. This OS and its ecosystem is still a fast-growing sapling, and I expect it'll be awhile before it is the stately tree that iOS has become.
With that in mind, I bought into the Winphone game last week, with the full expectation that I'd be carried forward for one or two generations of device and OS development. Assuming that Microsoft is trying to draw as many people as possible to their fledgling OS and ecosystem, I can't think of anything worse for them to do than to give the impression, or worse yet impose the reality, that people who buy in now are going to be frozen out in six or twelve months. That is just not the way to rally support to your come-from-behind effort! You need customers to feel that they're an ongoing part of something that's growing, gaining, and getting better fast. And they need to feel it strongly enough that their friends pick up on the vibe and jump on board too.
I thought this over last week before I bought the 900, and decided that it would be such certain suicide (with the general public) to refuse an update to a six-month-old phone, that Microsoft would never do it. I still feel that way, and I'm not too concerned about the rumors... though I AM puzzled by Microsoft's batten-down-the-hatches answers. It is clumsy for them even to cast doubt about this question.
Nevertheless, if Apollo doesn't come to at least the 900 (and other phones released with Mango already installed), I'll consider it the death knell for Windows Phone as a viable ecosystem. There is no way for that kind of attitude to succeed outside of the dedicated enthusiast base represented here. I have another upgrade slot on my family plan coming up this Fall, and if the 900 gets left behind, you can bet your last dollar that I'll absolutely burn that slot on an iPhone 5 rather than another Winphone. I won't throw the 900 in the trash, but there won't be any more app purchases, showing it off to friends, or any future Winphone purchases for me.