Well in theory all those things can be coded in software mode without DirectX (the linear filtered rotations, zooming etc) but a lot of them would be prohibitively slow on a CPU. The graphics hardware is much much more efficient at these tasks, so if you don't use DirectX and the graphics hardware and stick to the CPU, you can end up with 1-2 FPS fairly easily with these kinds of effects going on.
Games should still run fairly well on old computers. The Geforce 2 32mb graphics card is extremely old, and can still run Construct games at a playable rate, so long as you keep to "cheap" rendering - don't zoom the screen, just scale, rotate and use alpha blended sprites. Even very old hardware can do that just fine. If you're worried about performance on very old computers, completely avoid shaders - old hardware simply won't support them even if they can run DirectX 9 games. You can, however, use some shaders, and set them up to disable if the hardware doesn't support them - see Effects in Construct.
I don't see much use in implementing an older DirectX version - once you've got DirectX 9, even very old computers can run the games.