I did some calculations based on 2014 train congestion data)

• Between 7:30-8:30 on the Sobu Line Rapid, 63,120 passengers travel between Kinshicho and Shin-Koiwa (178% overcapacity), with nineteen 15-car E217 series sets (13 + 2 green cars) being shared with the Yokosuka Line.

• Between 7:30-8:30 on the Yokosuka Line, 35,680 passengers travel between Musashi-Kosugi and Nishi-Oi (192% overcapacity)

• Between 7:30-8:30 on the Nambu Line, 40,980 passengers travel between Musashi-Nakahara and Musashi-Kosugi (195% overcapacity), with twenty-five 6-car 209 series sets being used.

Each 6-car E233-8000 set can carry 924 passengers by design. If all 35 were in service at one moment, that 195% overcapacity would decrease to 127% overcapacity. Some of the 209 series sets are probably still in use, too.

Sobu/Yokosuka trains are already 15 cars long. It's not like JR East can make them longer. And adding more trains per hour might mess up signalling with trains too close to each other. With the recent problems concerning the E235 series' rollout on the Yamanote Line, it's probably fortunate that JR East refurbished the E217s anyway.

The Kofu area is not very densely populated (in fact, Yamanashi is the least populated prefecture in Honshu), it's mostly mountains. I don't see a demand to replace the 115 or 211 series anytime soon, I think the Niigata area really needed the E129 series especially with the Hokuriku Shinkansen.

Also, I think you're being too critical of JR East. The E217 was developed in 1994, and it's the oldest urban rolling stock in Tokyo. There are lots of 113/115, 211, 103, 201 and 207 sets in Keihanshin that are as much as 50 years old.

To replace the E217 series in the future (2030s/2040s), maybe JR East could scrap the green cars and make a new series of double decker cars, like they did for the Tohoku Shinkansen? Call it the E239-MAX series. I think a 15-car E239-MAX series train would go a long way to cutting congestion on the Yokosuka Line.