"Aluminum frames unlike steel ones have limited life-spans, hence the mountain biking axiom "Steel is Real"."
This is mostly correct. Except that even steel has a life span but it is much longer than aluminum. And, GENERALLY, steel does not have catastrophic failure. Back in the day when ALL race quality bikes were made of Columbus SL (or later SLX) tubing or Reynolds 531c or 753 tubing, all of these being steel, we road them until they began to "flex out", meaning that that thousands of miles of honking up hills had caused the steel tubing to become more maleable abd bend easier. At that time we knew we needed to get a new frame. Unless, of course, your team sponsor was nice enough to give you a new frame every year.
Ah, road bikes. Steel mountain bike frames won't typically wear out like older road bike frames will for a simple reason: They're overbuilt for endurance whereas road bikes were built with weight savings in mind. Of course they still have a lifespan, as all metal does, but a steel mountain bike frame will outlive a couple of generations of riders.