Not that strange, as GCC was an effort to a goal of developing an ecosystem of Free (as in speech) software. While FSF had sometimes made allowances for support

Not that strange, as GCC was an effort to a goal of developing an ecosystem of Free (as in speech) software. While FSF had sometimes made allowances for supporting non-Free software (whether non-copyleft open source or proprietary), these were always tactics in support of the longer-term strategy. Much like you might spend marketing funds on customer acquisition in the service of later recurring revenue. As RMS indicated, this strategy had already resulted in the development of C++ front ends for the Free software ecosystem, that would otherwise likely not have come about. At that time the boom in MIT/BSD-licensed open source software predominantly driving Web apps and SaaS in languages like Rust and Javascript was still far away. GCC therefore had very high leverage if you didn't want to be beholden to the Microsoft ecosystem (it's no accident Apple still ships compat drivers for gcc even today) and still ship something with high performance, so why give up that leverage towards your strategic goal for no reason? The Linux developers were more forward-leaning on allowing plugins despite the license risks but even with a great deal of effort they kept running into issues with proprietary software 'abusing' the module APIs and causing them to respond with additional restrictions piled atop that API. So it's not as if it were a completely unreasonable fear on RMS's part.