I don't think the aeroplane knows how many instruments are on board apart from a cg perspective. I'm pretty sure the wings will still fly no matter whether the DI is 20 degrees out.
Bear in mind that there are plenty of aircraft that don't have gyro instruments. That kills the DI, ASI and Turn Coordinator. For VFR flight in simple types, an ASI, Altimeter, RPM, Oil Pressure, Fuel level indicator, Slip and Compass are plenty for me.
If your flight requires more instrumentation ie radio Nav, of course you need more kit. If you're flying complex type then more engine instruments such as MAP are relevant.
It is true, though, that if you are used to seeing lots of kit in front of you, it can be disconcerting to learn that it's okay without. I had that experience a long time ago learning to use a DI for Nav, then moving to an aircraft with only a wobbly wet compass. You soon find that it's okay!