I have concern that a straight repeal could gain some traction and even have a chance to get through. Without the pressure of presenting a replacement, they could pass it off as a win, "we did what we promised to do" while continuing their empty promises of coming up with something better within two years of the expiration.
More importantly, as the replacement is being drafted over those 2 years, Democrats will be unwilling to support or help any bills as they'll still be upset about the repeal of Obamacare, which will then give the Republicans an out. Right now, the onus is perceived as being on the Republicans, but if they can change that message to, "Well, we've been coming up with ideas since the repeal but the Democrats are refusing to work with us, even though the deadline is coming up and will leave millions uninsured," they can spin the message to make the Democrats look like the cause of the problem (and their base and moderates will probably lap it up).
I would hope that enough Congress members would stand up against a nebulous promise of "We'll come up with something, trust us," but I also could see them so desperate for a short-term "win" and also petty enough to do something like this.