> Nothing stops someone from taking the free Windows Vivado and making it run on Linux

The EULA and the fact that the linux versior runs faster & has fewer bugs.

> just the device-dependent backend would be a major improvement and the frontend and optimizer could be shared with other toolchains

That's yosys and it's used by smaller commercial vendors.

> or reverse engineering then bitstream format for these FPGAs

Getting the timing is the hard part (+ good routing afterwards). The bitstream format has AFAIK mostly been reversed. 7 series has mediocre support , but US, US+ and Versal doesn't (probably because they're too expensive for personal usage).

It's a practical position. The era of Winmodems was a long time ago, and the hardware was terrible compared to what we have now. Today, SDR is a fun thing anybody can pick up as a hobby. If it doesn't work, I have three other ways of accessing the Internet. If your winmodem didn't work, you didn't have a smartphone or a tablet to connect to ChatGPT (well, Altavista at the time) with and look for help. Then, when they did work, they were really bad because the single core CPUs of the day didn't have multiple cores to have the CPU cycles to run the software end of the modem and do anything else at the time. Which meant if you were running a game (Tuxracer, perhaps? Linmodem support wasn't broad, but it existed) at the same time, you lost. That tends to cause people to not like the product.