Would you accept an NI only backstop?
Theresa May wouldn't; Johnson well might do. Except at present, he still has Parliamentary arithmetic to contend with and even with the DUP has a wafer thin majority. He only needs to lose next week's by-election and have one Tory MP resign the party whip and whoosh, the majority is gone.
Thatcher, Blair could get away with some of their actions because they had whopping great majorities. Johnson might think he's popular but thousands of people absolutely loath him.