We are seriously looking into that.
That said, the same caveats there will apply that applied to the old Simple Hub appliance. HomeKit control will not work on a non-Apple platform. Given it would be a Hub and thus only triggers are affected, for some people that’s not an issue. If you do use HomeKit for home automation and you want to tie that together with the rest of the world which Simple Control is uniquely able to do though, that’s an issue.
Also, cheap depends on what features of the Hub you use. Again referencing back to the old appliance, the horsepower in that was not sufficient to handle video transcoding for Remote Video introduced in V5. So while a Raspberry Pi build of Simple Hub would be nice for the basics, it would be missing at least HomeKit and Remote Video. A higher end box would be recommended. On the other hand, a Linux box strong enough to handle many video streams would generally be much cheaper than a macOS system. But certainly more expensive than an Apple TV 4K which hits the perfect sweet spot of low price and exceptional speed due to its dedicated Apple video decoding hardware.
So, again, we’re looking at this, but it’s not the holy grail. I think it would be great mainly for people who already have a Linux-based solution like a Synology NAS because for them it’s not a question of price, it’s a question of free vs not free.