This is not quite true; it is possible to "lose" the proper Russian accent.
I knew several people who spoke like that; without exception they were wives of Americans, who have lived in the US for 15+ years without communicating with other expatriates or visiting home (it was in the early 90's, when suddenly there were a lot of jobs in the former Soviet Union for people like that so they came back). The accent was not strong but definitely noticeable, mostly in the intonation patterns, and in the lack of vowel reduction (i.e. overpronouncing).
Also, once I learned how to pronounce English 'p' and 'k', I noticed myself putting those sounds into Russian words. .
Yet another issue is a particular (slight) accent, found in some old expatriate communities in the US (usually in older ppl who left, say, before 1985 and their American-born descendants).
These people are mostly Jewish, and probably had the accent even back home.
And of course all expatriates mix the English words and Russian grammar a lot