If i were to completely surround myself and live in japan without speaking ANY english...?

Question:

would I forget English (because I don't practice it) and become FLUENT with my Japanese?

Answer:

Eventually, yes, but only after at least a full decade of purposely denying yourself English, you will become fluent.
This happened to me when I moved from The Netherlands to Canada.
After 3 years of constant English, my mind changed from thinking in Dutch to thinking completely in English.