Nobody is forced to buy, in fact the old version should still work fine.
No, it doesn't. That's the problem. Fusion 3 is buggy on Lion and doesn't run at all on Mountain Lion .
Though there is some truth to your point. I'm guessing it is a non-trivial amount of work to support new OS releases. Perhaps it's even more work than the new features being added. Surely they shouldn't be obligated to support old releases forever. But a discount to existing customers sure would make me less likely to explore cheaper alternatives...
It's true on Linux as well. I upgraded my work machine from Ubuntu 11.04 to Ubuntu 12.04, and my VMWare Workstation 7 stopped working. (The kernel modules it uses are incompatible with newer kernels.) There are some third-party patches that are alleged to make it work, but I couldn't get them working quickly, so I just downloaded the version 8 beta instead. When that expires in October, I either have to pay for version 8 or retry hacking version 7.
I hear what you're saying, and I'm usually on your side of these debates.
But it's hard to use the existing version:
* they stop doing updates (bug fixes, security fixes, etc) on the existing version when they release a new one. And being a complex product, there are a lot of bugs to fix :)
* the existing versions don't work well with new OS X releases - Fusion reaches pretty deep into the OS, as I'm sure you know. I had to upgrade to Fusion 4 to make Lion work. Not sure yet about 4 on M.L.
The problem is Fusion is not really a mature product. It's a hard piece of SW to write, so the lack of maturity isn't really a problem by itself. It's just that they've effectively put me on a $50/yr SaaS plan without me realizing it.