It is interesting that one of the most powerful tools in an EEs toolbox has turned into a sales/marketing tool...
I would say > 90% of the EEs I know use LTspice. It is free, powerful, and it is easy to add models. Despite the (IMO) quirky interface (but fast, if you use it a lot), I think it made it to the top of the heap because they opened it up to third party models early on and did not put any limitations on design nodes, and they made the license pretty liberal in terms of usage. I'm sure some LT (now ADI) marketers are still losing sleep and gnashing their teeth about their failure to monetize this properly. However, I know that I have designs with expensive ADI parts because they were in the library, so I know they got a few sales out of it.
But, it's still Spice. Yes, LTspice probably has some proprietary algorithms, but I bet there are many ways around that. And, ADI/LT's product is not circuit simulators, it's ICs. I bet they are not real worried about Qorvo right now, because it not a big competitor in the IC realm. Maybe it will be someday, but not today.
Either way, suppose they lawyer up and try to quash Qspice. That would be expensive and what would it get? More ADI IC sales? I don't think they would spend the money to do this unless it started to look like Qorvo might threaten their core IC business. If that happens, i.e. if they actually feel threatened about ICs from Qorvo, they have bigger worries than circuit simulators.
In the meantime, Qspice looks really nice, but still rather buggy (though they seem to be working hard on cleaning it up). The C/VHDL capability might make it the killer app someday, but for now it is hobbled by lack of libraries.
Just my 0.02 in your favorite currency,
John