You're right that if there is sufficient water pressure and
the cold-water feed pipe to the bathroom is large enough,
flushing the toilet won't cause the temperature in the shower
to change noticeably. Another thing that would prevent the
problem, in the absence of pressure valves, is a separate
feed pipe to the shower which some houses have.

However, that's not a really useful answer for someone who
*does* have the problem of the shower temperature changing
when someone uses the cold water. Putting in a larger feed
pipe to the bathroom is an expensive operation. Increasing
the water pressure, if the pressure is low in the whole house,
is even more expensive. Installing a pressure valve in the
shower is a much less expensive way to solve the problem.

As for keeping the hot water heater temperature low enough
that you can shower with only hot water, you're right that
that saves money and reduces the risk of scalding, but this
has several disadvantages:

* It makes your dishwasher (if you have one) work much
harder and/or not get your dishes as clean.

* Your clothes washer (if you have one) may not get your
clothes as clean (if your washer does automatic temperature
control, this will probably only be true on the "hot"
setting).

* Your hot water will get used up more quickly if there are
periods of the day when you're using a lot of hot water.