You are right that there will not be a 5-4 reversal of Roe. There will be a 5-4 reversal of Stenberg. Then Casey. Then Doe-v-Bolton. By the time we try Roe again, there will be 6 or 7 votes against it.
That's how it worked with Plessy v. Ferguson, which was not actually overturned by Brown v. Board of Eduction. P v. F died a death of 1,000 cuts.