I don’t think pickleball is “little tennis”, but there are smaller versions of lots of other sports; indoor soccer, Arena football, softball, half court and 3 on 3 basketball, and so on. But golf already has that with par three and executive courses.
I can’t think of an example of any sport truly shrinking, with the larger, original version becoming secondary or ceasing to exist entirely. For that matter, I can’t think of an example where the popularity of the smaller version has even come close to matching the original.
The fact that I can’t imagine something doesn’t mean much, but I can’t imagine a significantly shrunken version of golf becoming dominant. I can, however, imagine golf more or less disappearing as the world gets more crowded, and hotter and drier, but if/when those things occur, the difference between current golf course sizes and the miniaturized size probably won’t matter a bit. I think it’s much more likely that the day is coming when you won’t be allowed to water your lawn than that there will be a pass given to golf courses under a certain acreage.