[imoreno]

It's hard to guarantee what will happen in 5 years. Even if the vendor was really principled, who will honor their support commitment when they go bankrupt?

Instead of a "guarantee that it will run properly in 4-6 years" you should be looking for "guarantee that you can find a workaround when it breaks in 4-6 years". Avoid lock in, proprietary formats and protocols, poor compatibility, requirement for exotic platforms... Many DirectX games work great in Wine. It's the ones that do crazier tricks that are harder to deal with.