It's pretty much guaranteed that any software defined radio you build today will be almost impossible to get parts for in 10 years, so your maintenance budget ends up requiring a software developer to upgrade things when parts become obsolete. At least with computers from the 1980s, the hardware is fully understood and can be much more easily repaired. There are already projects that have produced modern replacements for custom chips in machines like the Commodore 64, and there is at least 1 underway for the Amiga's chipset.