Yes, I am well aware of the strict codes. I don't know how it is specifically in St. Louis, but I believe in a lot of areas once you shut down an older structure for a period of time and then want to resume using it, you have to bring up to code.
If close the building and it changes hands and needs a new occupancy permit, sure. But that's not the case here. It's not really closed, just closed to the public, or no longer in use as a gate area. Off limits save for special occasions.
I'm sorry, but you are not going to convince me that leaving the existing terminal in place (largely unused) and not doing anything other than minimal maintenance will cost less in the long term as opposed to a phased demolition/rebuild of the terminals. There are numerous examples of old structures that require renovations costing a ton of $$$ to bring it into a state of good repair and the powers that be find that its cheaper to simply demolish.
Even the amount that's closed now is increasingly small. The tail end of C and the portion of D between E and the C/D security checkpoint are the only areas really out of regular service at the moment. And they're talking about remodeling more of C. B isn't in use as a gate area, but they're using it as a special event space. Good rent in that. If I needed a reception it'd be on my list. T1 isn't busy, but it's not the ghost it was a few years back. And it's largely remodeled and, save for the ticketing hall, probably not as old as you think. The concourses were largely rebuilt in the seventies and variously remodeled in the eighties, nineties, and otts. D was even entirely new in the early 80s. That's not a new building now, but it's hardly old. And that only makes it twice as old as T2. Honestly, save for the foundations, nothing in the concourses is really all that old. Honestly, the remodeling has, in some cases, been pretty close to the "phased demolition and rebuild" you describe. The portions that are in day to day use ARE in good repair now, so far as I can see and from all reports I've heard. And the very fact there are underutilized concourses has allowed that to happen. In a phased way. Close C, open D. Remodel C. Open C, close D. Wash, rinse, repeat.
I don't really understand why STL is so attached to the existing terminal when other airports like it are redesigning and rebuilding to bring the airport into the 21st century and better serve passenger needs. STL deserves better.
Seems like a two part question to me. Why do I like the existing terminal? It's beautiful. It's historic. It's unique. It was, so far as I can tell, the first truly modern airport terminal and influenced a generation of similar buildings, both in appearance and in design. There were a few later alternatives; Pan Am's Worldport at JFK or MCI and later DFW; but none that proved so enduring. The only major difference between T1 and a truly modern design is that the entrance to airside is on the baggage claim level and not the ticketing level as is more usual these days.
What would a new terminal bring? New service? Better amenities? Why? How? And at what cost? The amenities are pretty comparable to what I've seen anywhere else in the world., save possibly for buying tourist brick-a-brack. The food seems about the same. The seats are comparable. The planes are the same planes. The wait for security is minimal. That's really all I care about. Would love more planes. Tell me how a new terminal brings that? I fear we'd have a small glass terminal that would look dated in twenty years and more expensive tickets to pay for it. Yes, I'd like to keep what we have. This isn't MCI. You're presenting a false choice based, I believe, on mistaken assumptions.