I really don't intend to fan flames here, but this upgrade issue is something I see from both sides of the fence. I've worked as a photographer, a photo dealer and now as an Apple IT consultant.
You have to decide what your basic upgrade strategy is going to be. From my point of view, you have two options. You either stay on the current upgrade bubble and do relatively minor, but constant, upgrades in software and hardware, or you set up a system that works and run it until it becomes obsolete and replace it entirely as a major upgrade.
This is not at all unlike how you approach your car. Do you get a new model every few years, or do you buy one and run it until it drops? I'm not advocating either strategy here... but this is what you have to decide on.
The assumption that you're making is first, that a software upgrade is somehow separate from a hardware upgrade. They are absolutely, and essentially linked. You cannot assume that any, ANY software upgrade will not in some way require a hardware upgrade. This is why it's called a system. The second assumption that you're making is that a software company can somehow guarantee that their package will be current with any forseeable hardware configuration for a considerable amount of time. You're talking 4 years at this point? I would say, respectfully, thats unreasonable.
To go back to the basic strategies I mentioned, what you're doing here is using the "drive it into the ground" strategy, but then only looking to replace, er, the motor, for lack of a better analogy. In some cases you can do that, in some you simply can't, in some cases (for a car, anyway) it's downright dangerous. If you're going to run the system without regular maintenance, you have to expect that you will need to replace the whole thing at once.
Again, with respect, you've tried to have your cake, by buying the upgrade path (which a manufacturer like Phase really intends as a maintenance strategy, by the way) and to eat it too (by staving off any upgrades until it becomes absolutely essential).