>>1954420>>1954999They're old models from the Japan National Railways era where all the design and engineering were shared, inherited by each regional JR.
The rate of rail car replacement depends on how frequently trains are used and how fast they become uneconomic to operate and must be replaced.
Some old trains are remodeled extensively inside and out and operate as tourist trains, which has some limited shelf life compared to if new construction. It's nicely explained in
https://www3.nhk.or.jp/nhkworld/en/ondemand/video/2049101/ 
