Nothing in Xilinx's decision says you can't design/buy/support obsolete chips. You have today what you had then: ISE. Go for it!
If your product can stand moving forward, well, there's a new toolchain. Nobody is forcing you to use Spartan 2 for new designs but it you do, you will have to use the toolchain that supports the chip. What's the big deal? It's what everybody was using when the device was current. Realistically, all of the project files will already be compatible with ISE and certainly not Vivado (especially the constraints file).
ISE works, it is still viable, and if you insist on using old devices, it's the toolchain of choice.
I worked in an industry where computers couldn't be upgraded, compilers couldn't be upgraded and source couldn't be changed without a MAJOR verification process. It was too cumbersome to even contemplate. So, we stayed with obsolete computers, obsolete compilers and obsolete software because they were PROVEN to get the right answers for our design process.
If you take the purported $200M and 1000 man-years, you can get to $200k/man-year. Would I believe it would take more than 5 man-years to port the old devices? Well, when ratioed against 1000 man-years for the project, sure! I believe it could take 10 or 20 man-years to port the devices depending on how far back they went. And no matter where they drove the stake, people would be upset that their favorite device wasn't included.
$200k/man-year seems about right, if not a little light. Figure $150k for wages and 35% for overhead (employer social security, medical insurance, worker's compensation, paid vacations, sick leave, etc).