The great thing is that there are a ton of good cross-platform options available now, not just Unity. From commercial alternatives like UE4 to solid open-source

The great thing is that there are a ton of good cross-platform options available now, not just Unity. From commercial alternatives like UE4 to solid open-source libraries like SDL2, or just using OpenGL ES. If you have even the slightest inclination to release on multiple platforms, you should be building it as cross-platform from the beginning. There are very few reasons not to.