Just to clear up a few bits of “miss information”. Most strips are available PPR, and the numbers are in Navbox 99% of the time. Most have planning or legal use. Many are used for flight training on things like Eurostars and are considering expanding into GA training now the rules have changed. £20 for a landing fee…
With regard to sustainability, the strip world has existed for as long as I have been flying (1984). The only difference is there are now a lot more. People tend to lose sight of the fact that the average light aircraft will fly quite well out of 600m of Grass, without a lot of expensive infrastructure supporting it.
Rod1