I cannot follow the rest of your proposition.
Does a Doctor wait until someone's had a heart attack to come to the conclusion that the person has a cardiovascular problem?
Most cardiovascular failures, like most ignition system failures, are caused by problems that start small and get bigger, but can be detected and addressed when they're small. That's why doctors check, among other things, your blood pressure. In some cases your doctor will connect a bunch of wires to you and put you on a treadmill.
A "healthy" piston aero engine with duplicated, "healthy", ignition systems is able to run at high power on one of those systems. Indeed, if it's a certified engine, it's part of the certification basis.
When your engine is at high power at altitude, it's on the cardiovascular version of a treadmill. If you have an engine monitor connected to it, you're measuring many of its vital signs.
If any part of your ignition system has any small problem that is slowly getting bigger, the most likely time at which it will
first manifest itself is at high power, high altitude, LOP. The most effective way to narrow the problem down is to run each mag separately in those conditions.
(Other developing problems, like small changes in static timing, will often first manifest themselves in unusually high CHTs at high power during climb.)
These problems can be diagnosed and fixed
before they become the equivalent of a heart attack.
The most recent real-life in-flight ignition system problem I had only manifested itself at 9,500'. I diagnosed the problem with a magneto check. The problem disappeared passing 8.500' on descent and the system tested perfectly on a "standard" ground magneto check. But the problem was still there, was fixed, and therefore was prevented from getting any worse and doing more damage to the ignition system.
Almost all ignition system problems don't need to be left to develop to such an extent that an engine "fails" a "standard" ground magneto check. Once it's got that bad, it's almost certain that the engine hadn't been performing as well as it should have, for a long time beforehand.
In the case of the perceived risks of doing an in-flight mag check that shows one magneto is "dead", think about how it's possible for the aircraft to have got to that state in the air in the first place, without the pilot knowing. If it's
really the case that the aircraft had a genuinely random in-flight failure resulting in a completely dead magneto, even an aircraft without an engine monitor is going to "tell" the pilot something is "not quite right". And then the pilot's going to do, what? A mag check, among other checks...
But I get it now: It's dangerous to do an in-flight mag check on an engine whose monitor and trend data shows it's currently healthy, but not dangerous to be blissfully ignorant of the fact that you're flying along with a "dead magneto".
Eddie: what type of engines fitted to what aircraft, precisely, stop rotating in flight when the mags are switched off or the fuel runs out? I'm yet to find any aircraft piston engine that can be stopped rotating in flight, other than by switching off the mags or fuel and stalling the aircraft.