I'd pay four figures for a significant bitcoin settlement transaction if that was the market price.

That's a high price because it starts to become HNW and institutional only even in the developed world. Nothing necessarily stops it from getting that high but the downward pressure at that point would be tremendous because there would be so many alternatives that are reasonably secure, but also faster and more private and more programmable, but for 1/100th or 1/1000th the cost as being on the main chain.

To put it into perspective, if in the future there is higher adoption and just ten million entities in the world (let's say 10% of businesses and HNW people) want to do an on-chain bitcoin transaction on behalf of themselves or their customers per week, they can't. There's not enough blockspace for that. Even they have to use on-chain transactions less frequently, make use of large Lightning channels, make use of other L2s, or make use of whatever soft fork methods exist at that time.