50-70 years is the typical harvest time in Oregon. This works out fine because we have a large amount of forest, douglas fir is the most productive harvested timber by acre, and they do not harvest the whole forest at once. They typically harvest 1 square mile sections at a time.

Here are some pictures to give an idea of the logging pattern: https://imgur.com/a/grid-grid-logging-oregon-ewNJL2e

That just makes it slightly more expensive than a faster growing timber, most of the cost of timber production is labour, machinery depreciation, transport.

Steel is 100% recyclable, indefinitely, and energy source agnostic.

I believe in this case they aren't waiting for it to mature. They use what's called LVL (laminate veneer lumber) which is basically thin sheets of the wood glued together (think plywood but thicker) and then recut into dimensional sizes. The end product is both stronger and straighter than conventional lumber. And because you don't need a large cross section (almost literally any size will do), you can have a pretty short planting -> harvesting cycle.

Sure, and there are tree farms that have been around for longer than that.