why Israel can't go back to 67 lines

The dangers of relinquishing military control of the West Bank are as follows:

An Arab army will attempt to sever Israel at its narrow waist along the coastal plain.

Palestinian forces - regular or irregular - will infiltrate along the line, and given the tiny distances they'll be able to reach Israel's main cities within minutes and wreak havoc.

Palestinians will be able to shoot directly at numerous targets in Israel's populous heartland.

Palestinians will be able to shoot mortars and short-range rockets at numerous targets in Israel's populous heartland.

Israel will lose its ability to collect human intelligence about terror cells in the West Bank.

Rather than controlling the West Bank, Israel will have to defend itself along a long and twisted border much of it in hilly terrain.

Israel will lose most of its control over the aquifer that supplies much of the water to the coastal wells.
The Palestinians will have the legal right to demand some of the water of the Jordan Basin.

These threats are of varying quality. The first, regarding an Arab army, can be fended off through two measures. First, the Palestinians will not be allowed to have a full-fledged army. If they ask the Europeans, this will be a blessing for them, since armies are extremely expensive things to have, but if they insist having an army is essential to sovereignty they should be reminded that Germany (both of it) was allowed only a limited military between 1945 and 1991, and got along quite well, and Japan's military was also limited post 1945. So no, having an army is not an essential prerequisite for sovereignty.

Second, Israel demands a military presence along the Jordan River, to the east of the West Bank. This presence is directed at anyone to the east of Palestine who might be tempted to use it as a launching pad for an invasion of Israel. There is total unanimity among all Israel's security types that this presence is essential, though Netanyahu has recently been hinting it need not require Israeli sovereignty. Perhaps the Jordan Valley will be sovereign Palestinian territory in which Israel has contractual rights to a military presence. I admit I'm personally skeptical. Modern armies being the cumbersome things they are, I don't see how one could arrive on the West Bank suddenly, unannounced, and launch an attack on Israel. Not to mention that no Arab army has tried the full-fronted assault method since 1973, probably for the good reason that it's a harmful exercise. In any scenario Israel will need a powerful and threatening military for the first three or five generations after making peace with all its neighbors, but I don't see why a few thousand troops along the Jordan make much difference. There's a major road down there from Jerusalem, and another can be built from the north, and if there's to be a war IDF forces will be there long before Iraqi or Iranian or Emirati divisions arrive.

Water: this is a serious matter, but ever less so. At the moment we're preparing to lay the fifth major pipeline from the coast up to Jerusalem (if I'm not mistaken), which will be unusual in that for the first time it will draw its water not from coastal springs but from desalination stations. There isn't enough natural water in Israel/Palestine for the 12 million people who already live here, and there's not going to be any more, either. Israel already operates major desalination plants, while holding the world record for recycling water; this trend will have to continue no matter what. I don't have the exact numbers at hand, but Israel already supplies some of the water the Palestinians use, and will probably supply more as their numbers grow, no matter who controls them politically. This means water will be a Palestinian weakness, not a threat against Israel. Anyway, the entire subject is one that can be resolved with money, and need not cost human lives.

Which leaves us with the various threats of low-level Palestinian violence. These are serious. In 2002-2004 Israel needed to reoccupy the entire West Bank, re-build its intelligence sources and networks, and also construct the security barrier; only then was the bloody 2nd Intifada defeated. Its ongoing control is the reason no kassam rockets or mortars are shot from the West Bank, while many thousands have been shot from Gaza. Moreover, only a fool, or perhaps a Swedish foreign minister, would believe that by signing a peace agreement with some Palestinians, there will remain no Palestinian individuals or groups willing to shoot at Israeli civilians from the shelter of civilians towns and villages; those Swedes and other EU fellows will conspicuously not fly into Ben Gurion airport if they ever remotely fear that their plane could be shot down as it comes in to land at the airport which is within range of Palestinian gunmen with easily portable shoulder missiles. Until someone comes up with a way to assure Israel this danger is not acute, I don't see how it will relinquish military control of some sort over the West Bank. Which is not to say that Israel might not move all its civilians back to a line, say that of the barrier. Which brings us to the matter of the settlers.

The real reason Israel insists it cannot go back to the Green line is a combination of security to the east of the airport, and the existence of large settlements, most of them quite close to the Green Line. No official maps have ever been made public, obviously, since the negotiations have never reached completion,