They could, but why? There's no money it in. Open sourcing .NET means they get contributions and users who would never touch a closed source core. They also ma

They could, but why? There's no money it in. Open sourcing .NET means they get contributions and users who would never touch a closed source core. They also make money from things around .NET and from cross-selling products that tightly integrate with .NET. They don't currently make any money from AoE2. Being as old as it is, it's not for sale new so when it does change hands, it's used or illegitimate. Open sourcing the engine helps the people who already handed over money but it's not going to draw in any more legitimate sales for the assets (which are usually non-free in something like this). And there are licensing issues. If they licensed used some audio middleware or video playback stuff (very common in games) they almost certainly don't have a license to redistribute that in source form. That means either paying more or changing it out. Either way that's money they have to spend on this thing that isn't making them any money. Not to mention there are already things like openage and 0ad which replicate or replace the engine with a ground-up free implementation. Rather than do any of that that, Microsoft commissioned a new engine (and assets) to release AoE2HD. They're getting a £15 for everybody who wants to play their old game on new operating systems. That's just the way you make money from old games these days.