They are rushing into it because they want a bill signed into law well before the end of the year. Lawmakers want this bill done before tackling the budget in the fall, the corporate tax cut and most of all the midterm election fundraising and campaigning which will go into high gear after year end.
After all the negotiating and compromises between the House and Senate, republicans will vote for the bill regardless of what it contains because they can not afford to go back to their constitutions without claiming they repealed Obamacare, which of course is a lie.
Obamacare will remain the the major healthcare law. The legislation the republicans are creating is just amendments to Obamacare touching only the parts the law which impact the budget. Everything else remains the same.
If republicans really wanted to fix the American healthcare system, they would wait till they have the votes to repeal Obamacare and create their own healthcare law embodying their vision. Instead they decided on a quick and easy fix which is proving not to be so easy or quick.