.NET proper has been OSS for a year and a half now (in the form of .NET Core), and .NET Micro Framework, which was specifically created to target resource-constrained devices, has been available under Apache 2.0 since 2009. Since parts of Mono have been available under MIT all along, even if Unity had some reason to favor Mono over the others, then they could have replaced the GPL/LGPL bits on their own.
It's likely that Unity hasn't done any of these things because they need support. This might help, but I'd be unsurprised if the Unity situation stayed the same as it has been for the last few years.
It's likely that Unity hasn't done any of these things because they need support. This might help, but I'd be unsurprised if the Unity situation stayed the same as it has been for the last few years.