"Are some of these guys who are out there running BIG horsepower through 20+ year old pumps just asking for trouble?"
I don't know about the 'asking' part, but, I do believe that there are alot of people who don't realize they should throughly check out the quality of their suction piece (Density, Core Shift, etc.) and take into account their age and the fact that AL Casting have a 'Memory' regarding Fatigue before they attach Big Hp to them. Attention should also be paid to the age and quality of bowls, droops and nozzles when entering into the realm of Big Hp as they are also cast AL.
Last year we blew the side out of our old 1978 Berkeley suction piece ... replaced it with a Dominator due to the casting being much denser & thicker and its having the side wall ribs which provide more support. I feel comfortable with the Dominator piece up to say something like 2,500 Hp ... For Hp above that and when we go there, we'll be running a Billet Piece.
Another thing that puts a suction piece under much more stress than it was originally designed for is adding a PopOff Valve ... When a PopOff goes off it sujects the casting to 'Quite A Bang', much the same as hitting the casting on the inside with a heavy sledge hammer. The mounting points of the PopOff should be carefully inspected and checked for cracking after Every Run