That is a big assumption. They would still need to get a work visa and have the desire to leave. There is a lot of culture at play as well, which would still be there if everyone in Japan was a native English speaker. English is certainly a valuable skill for traveling and moving abroad, but I hardly see Japanese people as trapped in their own country. According to the Ministry of Foreign Affairs of Japan there are 1.3 million Japanese citizens living abroad.

Unfortunately I could only find statistics information in Japanese.

http://www.mofa.go.jp/mofaj/toko/page22_000043.html

http://www.mofa.go.jp/mofaj/files/000162700.pdf (page 11)