The Republicans in Congress know they have a brief window to derail Obamacare before it ultimately proves itself in the marketplace. They understand that only now, while the ACA is still finding its legs, can they kill it. Give it a chance to run and it'll be too late.

If your goal is simply to destroy something, then it makes perfect sense that you'd not be worried about what to replace it with. "Repeal and replace" represents the logic of people motivated solely by politics. 

It doesn't bother Trump or McConnell one bit that they're playing Russian Roulette with the health of hundreds of thousands of people living on the margins in this country. That's a small price to pay for wrecking Obama's legacy.