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.