Well, Boris Johnson wants to be Prime Minister for the long haul, not just to get Brexit done. He won’t want to sacrifice the job he has aspired to from childhood over Brexit.
"For whosoever will save his life shall lose it; but whosoever shall lose his life for my sake and the gospel's, the same shall save it." -- Mark 8:35 King James Version (KJV)
If he gets BRexit done, he will win the next general election, regardless of what Parliament does.