Why is it necessary to formate, aerobat and stress these old aircraft?
Most of the manoeuvres these "old aircraft" do are well within their 'tolerances' - e.g. the BBMF display is very sedate compared to some privately owned warbirds. If a private warbird owner only wants their aircraft to be flown with kid gloves that is their choice - others are quite happy for them to be flown balls to the wall.
Many of these old aircraft are in better condition now than when they first rolled off the assembly lines 70 years ago.
If it wasn't for the 'industry' behind restoring these "old aircraft" most of them would still be sitting on a steel pole and we would have a handful of Spitfires still flying, rather than the 45 worldwide (as of May 2011).
There were 164 airworthy P-51s around the world, sadly one less for now but the industry can work miracles these days.
The sad loss of the B-17 "Liberty Belle" in Ohio a couple of weeks ago, happened on a straight forward flight from A to B after a fire caught hold after takeoff.
Would your ideal world have all of these "old aircraft" gathering dust in museums or hangars never seeing the light of day?
The P38 crash was a classic example of what not to do, consequently we have no P38...
"What not to do"? I'm sorry - do you know something about the cause of the accident that the AAIB doesn't?
The closing part of the AAIB report stated -
It is known that the pilot was a very experienced display pilot and produced high quality, aesthetically pleasing displays. There is no evidence to explain why the aircraft entered the second part of the final manoeuvre in a less than optimum pitch attitude which developed into a significant downward trajectory. The possibility of a temporary restriction to the flying controls (especially the roll control), or some other form of distraction of the pilot, could not be dismissed.
Thankfully this time all involved walked away.