I guess that time is over, and that's sad, I just don't feel like it has to be.
Maybe the declining population is just part of it all, there is just less incentive to go bigger, on the other hand, even 100,000 million people on an island the size of Japan is a lot so I'm not sure that's it either.
It is sad for the people of Kyoto though, because over tourism can really rip the heart out of a city for the locals.
As another poster commented, there are people who still build with traditional materials and methods. The temples are made of wood and have to be renovated. Some are completely rebuilt, symbolic of the transitory nature of the material world. Enryakuji is undergoing renovations and they had to completely cover it with a metal shed while they work on the roof. But it's still open and you can still visit either as a sacred site or to learn about the traditional methods. It is supposed to finished in 2026.
As far as building new temples. Those monasteries had thousands of resident monks. They were significant military powers they were so populated. Even though the overall population of Japan has grown, far fewer people want to live that life. But again, there's no shortage of temples.