You're kind of conflating several different points.
The ability for an application (eg, Office) to install side-by-side with other versions of itself is entirely different than its Backwards Compatibility (eg: ability for current versions to open and/or save Office 2007 format). In fact, because of the latter, provided they did a good job at it, the former is arguably irrelevant.
I'm not familiar with Office/Outlook/Access's APIs and their history of backwards compatibility, but given that Microsoft usually does a good job with BC, I'd tend to suspect the other vendors (IP phone/ERP software) are at fault. In my experience, there's a good chance the only real incompatibility is with the vendor's installer or some version check that prevents it from working, as opposed to really not actually working, say, because Microsoft removed some API they used.
The rest of what you're talking about really points to some other disfunction, or bad or unlucky decision.
Did you buy an IP phone system expecting 20 years of use from it? Why can't you get updates -- is it because you don't want to pay for maintenance, or did they go out of business?
The cascade effect you're talking about here though highlights something else: So many companies do things they think are saving money, but actually cost them more.
For example, they can't go to the latest IP phone software, because they need to buy a hardware upgrade that will cost $x, so it means they have to use old version of Access that uses old MySQL which means other development team spends $y extra working with that version (or paying for custom dev for other products to work with it, etc). Did anyone actually evaluate which is larger? The decision to "save" $20k on a phone system upgrade could easily cost $50k or more for several months of a developer's time.