How could we get a situation, with any PoD after 1815, emerge where the Great Powers let the Ottomans crush any Greek rebellion in the 1820s, the Turks win, and the Turks get at least a couple decades before any repeat rebellion.
Austria under Metternich would probably have been content for the Greeks to get crushed. Prussia probably did not care. The three intervening powers were Russia, Britain and France.
Russia under Nicholas I was somewhat conflicted, on the one hand opposing revolutions as a matter of principle but on the other feeling solidarity for the Orthodox Christian rebels. In the end Nicholas came down against the Ottomans and in favor of the Greeks.
Britain and France, going through the Romantic Era, were sympathetic to the Greeks based not on their knowledge or admiration of contemporary Greece, but on the reputation of classical Greece. That and sympathy for them as victims of Ottoman brutality.
But what could counter these forces such that they all let the Ottomans put the Greeks down and hold them down?