How do I know this will happen? It already has. In this article, Hamas leader Mahmoud Zahar has already pushed beyond the 1967 borders, questioning why the negotiations don't involve the 1947 Partition borders. Of course, Zahar ignores the fact that the Palestinians rejected the very same partition after the UN proposed to divide the land. Now that the 1967 borders are essentially the starting point for negotiations, and there is an explicit endorsement of some politically defined border, Zahar and other Palestinians will ask "Why June 14, 1967? Why not May 13, 1948?" Indeed, this type of negotiation strategy is entirely consistent with creating absolutely indefensible borders for Israel with the intent of bringing about a final confrontation in which Israel's position is so thoroughly compromised that even the IDF cannot prevent catastrophic causalities.
Indeed, I would challenge Mr. Obama to answer that question. Why does he choose the 1967 borders and not the 1947 ones? Since Mr. Obama is agreeing to peg Israel's future borders in a geographic setting that led to two wars within 20 years (1956 and 1967), as well as numerous border conflicts and significant bloodshed in Jerusalem, why would he be uncomfortable with the 1947 borders, mandated by the UN? The very notion that the June 4, 1967 borders are to be some sort of starting point with "limited land swaps" presupposes that such a border is viable. Although successive administrations have used the 1967 borders as a starting point, to a degree that it has became almost dogma to base all negotiations on those borders. Why? What makes the June 4, 1967 political lines remotely viable or logical for the creation of Palestine. Let us consider the fact that the West Bank and Gaza were controlled by two different countries (not Palestine) from 1948 to 1967. Indeed, Israel's lack of strategic depth in June 1967 was big reason why Israel took the offensive to assault the Egyptian air force in the war's first day, knowing full well that any significant military penetration by the Jordanians would endanger Jerusalem and even Tel Aviv. The existential threat to the Israelis would have been far more real had the Jordanians possessed a more formidable fighting force. At the time, the Jordanians were weak allies to the Egyptians, while a more militant and belligerent force in the West Bank, say Hamas, would pose a significant threat to Israel's population centers.
Of course, it's much easier to say buzzwords like "1967 borders" and "land swaps" without considering the history that led to those borders being discussed in the first place. The very reason why we are discussing "going back to the 1967 borders" is that those border were not viable in the first place. Territorial holdings changed precisely because 1967 Israel did not feel secure having a belligerent country, even one as weak as Jordan, holding territory so close to its population centers. When one looks at Israel's history, it is easy to see how quickly its neighbors seized advantage of strategic land positions to attack Israel. The reason why Israel was so focused on taking the Golan Heights in 1967 was because Syria used the Heights' strategic position to launch rockets and missiles into the Upper Galilee. To demand that Israel now give such lands back to its enemies, while the same enemies give up nothing but promises on paper, is to blind oneself to history.