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The Saudis are already destroying old buildings:
Sami Angawi, an expert on the region's Islamic architecture, said 1,400-year-old buildings from the early Islamic period risk being demolished to make way for high rise towers for Muslims flocking to perform the annual pilgrimage to Islam's holiest city.
“We are witnessing now the last few moments of the history of Mecca,” Angawi told Reuters. “Its layers of history are being bulldozed for a parking lot,” he added.

Angawi estimated that over the past 50 years at least 300 historical buildings had been leveled in Mecca and Medina, another Muslim holy city containing the prophet's tomb.

Wahhabism, Saudi Arabia's dominant doctrine which promotes a strict narrow interpretation of Islam, was largely to blame, he said.

“They (Wahhabis) have not allowed preservation of old buildings, especially those related to the prophet. They fear other Muslims will come to see these buildings as blessed and this could lead to polytheism and idolatry.”
http://www.islamicpluralism.org/467/dr-sami-angawi-on-wahhabi-desecration-of-makkah

However, the reason that ISIS want to destroy the Kaaba is that they believe that the end of times, Day of Judgement or Qiyamah is here. Read more here http://en.wikipedia.org/wiki/Islamic_eschatology

It is difficult to understand that they can believe in this, but they do, and they will kill for it.