DirectX 9 actually makes a lot more possible than you probably realised. I do understand that for retro style games you don't really need anything special, which is why we have an SDL renderer in the works, that runs only in software. However, it won't support any of the following:
- Linear filtered rotations/scaling/positioning
- Zooming or rotating the display or individual layers
- Layer effects/opacity/filters
Even having a game with large objects, or just lots of alpha channelled objects, would run unacceptably slow without DirectX. The CPU can draw graphics, but it's pretty poor at doing it - which is why DirectX is so crucial for having good performance 2D games. Still, if all you're interested in is stuff like early console games, the SDL runtime should suit you fine and also run on more people's computers. It should be done before the 1.0 release of Construct.
Also just to note - DirectX 9 can run on pretty much any computer. You just need to run the DirectX 9 installer, and Construct games will most likely work.