To add why an open-source toolchain would be useful:
It lowers the barrier to entry for FPGA development significantly. The Papilio project, for example, tries to build an "Arduino for FPGA" and runs into the problem that a full ISE or Vivado installation and corresponding license is needed. This aggravates the primary problem that the Arduino has solved for microcontrollers, easy entry.
Also: Incremental builds, partial reconfiguration, porting to macOS. I'm not sure which of these Vivado has solved. With an open-source toolchain, those interested in these features at least have a chance of building them themselves.
It lowers the barrier to entry for FPGA development significantly. The Papilio project, for example, tries to build an "Arduino for FPGA" and runs into the problem that a full ISE or Vivado installation and corresponding license is needed. This aggravates the primary problem that the Arduino has solved for microcontrollers, easy entry.
Also: Incremental builds, partial reconfiguration, porting to macOS. I'm not sure which of these Vivado has solved. With an open-source toolchain, those interested in these features at least have a chance of building them themselves.