I've actually seen stands made from concrete and have books explaining the process. You don't have to not have plans to move. You use sheets of plastic on the floor before you begin the process. Most use concrete supports with 8" I-Beams as runners.

Another point about oil rigs is that they do rust, constantly. They have special teams constantly replacing the bits. I can work on the rigs in my trade replacing pipes and make a good living if I so choose. The supports for the rigs are over engineered. Meaning they are sacrificial as was stated. Because of their heft, they begin to rust immediately. When enough rust is accumulated it can actually protect the remaining metals from rust producing oxygen. The difference is that we use thin walled hollow tubing to build stand with. Essentially it will never get to the point of protection without being eaten through.

A problem with stainless is that you would have to use 316L to produce a stainless that won't rust. Many 303, 304, 308 grades contain iron. When you reach 316 all of the iron has been completely removed and you finally reach "medical grade" stainless. Each grade is considerably more expensive. Not to mention the difficulty in finding a good orbital welder for this "soft" metal. Ever wonder why your "good" chef knives rust in your kithen sink? Because they must contain iron to hold an edge. They are typically 304 stainless.

Another good option, albeit quite new, are plastic plywoods. They look very much like wood and work the same way. They are stronger than wood and offer superior break down protection. Wood has been the standard and with some good protection should last a very long time. If truly in doubt, coat it in marine epoxy. You could even go as far as coating it in marine fiberglass, much in the way TexReefer built his 90g tank entirely out of plywood.