Although a few leaders have been assassinated, the main problem would be unpopularity and being discredited, thus having one’s career ruined. That would be true even if the rivals attacking you would be totally cynical themselves and thought the same way as you did.
This also applies to countries. Weaker states and groups had to keep their mouths shut and yell the slogans even louder. In this regard, Jordan comes particularly to mind as well as Lebanese Christians, among others. Of course, Iranian and Turkish leaders also rejected the destroy Israel notion, not being Arab nationalists. Now that they are governed by Islamists, however they have joined the chorus.
Here is a very brief history of this issue.
Pre-1949 period: The Palestine Arab leadership and the governments of Syria, Iraq, and Saudi Arabia engaged in True Belief, assuming that Israel would never come into existence or be quickly wiped out. King Abdallah of Jordan correctly assessed that Israel would survive, did not use the issue demagogically, but could not resist the peer pressure to join the war. The Egyptian government did not want a military conflict but could not resist peer pressure and engaged in cynical manipulation but the Muslim Brotherhood took a true belief stance as did lots of political forces in the country.
Arab Nationalist Era: In the shadow of the 1948 debacle, Arab nationalists argued that Israel only survived because of Western backing and the weakness/foolishness of the old regimes in their own countries. By creating strong central governments, modernizing, their own ideology, building up their own armies, getting Soviet assistance, and helping guerrilla/terrorist groups, the new regimes argued and believed that Israel would be wiped out.
For the Egyptian Nasserist regime and the Ba’th Party governments that ruled Syria and later Iraq, the Israel card was part of their hand for trying to seize hegemonic control in the Middle East. There was no contradiction between their true belief and their cynical manipulation. In this atmosphere of hysteria, only the Jordanian monarchy resisted though the Persian Gulf Arabs were pressed into giving more money by peer pressure.
The defeat of 1967 showed that the Arab nationalists couldn’t do much better than the old regimes but did not really change attitudes. Only gradually, through the 1970s and 1980s did it become apparent that any destruction of Israel would have to be longer term. In the interim, true belief continued to flourish but Arab states became more cautious. They also sought to use indirect means—Palestinian guerrilla/terrorist forces from the PLO and other groups—to bring about Israel’s downfall by sabotaging it socially and economically.
One could argue, however, that the proportion of cynical manipulation to true belief increased. In the late 1970s, Egyptian President Anwar al-Sadat put Egyptian national interests to the fore, abandoned regional ambitions, and made peace with Israel. The regime largely, though not completely, abandoned true belief and cynical manipulation while also defying peer pressure. The opposition rejected this stance but could do nothing about it.
With the collapse of the USSR and Iraq’s defeat by a Western coalition in Kuwait, the leadership in the Arabic-speaking world had largely abandoned true belief or, more accurately, switched it to a long-term proposition. Israel would be made to fall but it was not clear precisely how. And Arab regimes were unwilling to take many risks or devote as many resources on the issue. They had their own problems, notably a rising threat from Islamism.
“Peace Process”: The Palestinian movement followed a different course and in doing so had some influence on the states. It advocated the creation of a Palestinian state that would not be bound or limited by any commitments to institute a second stage, using the territory and assets it possessed, to wipe out Israel. Until 1993 it was unable to make even the minimum steps necessary to bring this about. Yet while a handful of moderates and a larger group of cynics and those seeking economic benefit were ready for a deal with Israel, the overwhelming majority of the political leadership wasn’t.
Briefly, they wanted to follow a two-stage solution through a temporary two-state “solution” but were unwilling ultimately—as seen in the 2000 Camp David meeting—to take the compromises and commitments necessary to get a state. The creation of Hamas put peer pressure on them.