Collins is the quintessential early 21st century GOP 'moderate'. But Cotton and Paul definitely are not. Far from it, though Paul's politics are certainly as idiosyncratic as they are consistently rightwing. But look at their states: Arkansas and Kentucky. These are both poor states, both states which took Medicaid expansion and both saw huge increases in the number of enrollees who got health insurance under Medicaid expansion. This doesn't include insurance through exchanges.
Total Medicaid/CHIP enrollment went up 69% since Obamacare in Arkansas. In absolute numbers, enrollments went up over 385,000. That total went up 102% in Kentucky. In absolute numbers, that was over 620,000 new enrollees. These are needless to say big, big numbers. Kentucky has by far the biggest increase of any state on a per capita basis and Arkansas isn't far behind. I do not think it's any accident that these two happen to be hold outs. (Maine's Gov LePage vetoed Medicaid expansion; but more than 87,000 Mainers have insurance the ACA exchange.)
Whether these senators will stick to their guns and effectively block repeal absent replacement I don't know. I'd say be skeptical. But it highlights another point. These used to be very Democratic states, in significant measure because they're generally poor and benefit a lot from federal programs like Obamacare - and earlier ones like Social Security, Medicare and so forth. Both have shifted hard to the GOP as cultural and social issues have grown more salient and as the urban/rural divide has aligned more closely with partisanship.
But this could be a case where these underlying socio-economic realities come back to the surface. There are a lot of people benefiting from Obamacare in these two states. I think Cotton and Paul know that.