In a correctly plumbed system there should not be a significant pressure differential on the hot and cold sides. When done correctly (upsized feed from the utility) with balanced loops or a manifold flushing the toilet shouldn't effect shower pressure or temperature at all.
I can testify to that because in our house it works that way. The only time a shower is affected is when filling the large bathtub - which fills much faster than in most homes because it has a very large feed but that's the tradeoff since there isn't enough volume from the utility in that one case. Otherwise people can flush toilets, use the kitchen sink, wash clothes, etc and showers are not affected whatsoever.
Another factor is insulation - even many modern homes don't insulate the hot water pipes so even with an instant hot water heater that responds quickly you still have to pay the lost heat cost of warming the pipes and everything the pipes touch before the temperature fully stabilizes. This is a cost-saving measure - there is no reason the pipes couldn't be insulated. If you throw in a recirc pump then you can have instant hot water at every tap with very low energy cost.
Even if the pressure does not change, the temperature definitely does. Currently, cold water is basically icecubes blasting out the shower head. Warm/hot water, on the other hand, is sometimes affected by a heat exchanger (district heating) malfunction.
I can testify to that because in our house it works that way. The only time a shower is affected is when filling the large bathtub - which fills much faster than in most homes because it has a very large feed but that's the tradeoff since there isn't enough volume from the utility in that one case. Otherwise people can flush toilets, use the kitchen sink, wash clothes, etc and showers are not affected whatsoever.
Another factor is insulation - even many modern homes don't insulate the hot water pipes so even with an instant hot water heater that responds quickly you still have to pay the lost heat cost of warming the pipes and everything the pipes touch before the temperature fully stabilizes. This is a cost-saving measure - there is no reason the pipes couldn't be insulated. If you throw in a recirc pump then you can have instant hot water at every tap with very low energy cost.