It wasn't so much that the WW I Allies were "overreaching with The Treaty of Sevres"; rather, it was the fact that the WW I Allies had underestimated the rising wave of Kemalism and a new Turkish Republic movement which still retained the ferocity, ruthlessness and steadfast determination of the centuries old Ottoman Empire.
From the Greek perspective, The Treaty of Sevres, was initially, a victory for Greece. Beginning in 1919, Greece had regained-(though to a limited extent), the ancient region of Ionia in Western Turkey/Aegean Turkey, including, the centuries old city of Izmir/(Smyrna). It had been over 500 years since the Greeks had territorial control over this historic region and by 1919, the return of Ionia and Smyrna to Greece was, in a way, a near completion of the 100 year Hellenic struggle over Ottoman colonial rule. However, the return of Ionia and Smyrna to Greece only lasted 3 years and it culminated in a disastrously fiery end for the Greeks. This devastating event was titled, "The Smyrna Catastrophe" whereby the Greek citizens of Smyrna fled for their lives, due to the rising onslaught of rabidly anti-Greek Turkish Nationalists led by.......Mustafa Kemal "Ataturk". By September, 1922, the city of Smyrna, was "ethnically cleansed" and the city's entire Greek population fled the city's fiery destruction- (sparked by Turkish Nationalists).
From the British and French perspectives-(regarding Greek Ionia/Aegean Turkey), they lost a key geopolitical Greek ally. With the loss of Greek Ionia to the Turkish Nationalists, the Eurasian Aegean region, was now officially divided between an enfeebled Greece and a powerful Post-colonial Turkish Republic. In other words, the British/French interest in weakening and punishing the Turks for co-starting the First World war, actually emboldened and strengthened the Turkish Nationalist cause, resulting in, the present-day geography of the Turkish nation-state.
The WW I Allies naively and erroneously believed that the Turkish Nationalists would be satisfied with a state that was confined to much of Central Anatolia. They absolutely underestimated the ability of the Turks-(despite the birth of the Young Turks movement a generation earlier) to transform their cultural and political identity into a Post-Ottoman Westernized Republic.
So again, it wasn't necessarily an overreach on the part of the WW I Allies, but more of an underestimation of the Kemalists and the Turkish Nationalists vis-a-vis, The Treaty of Sevres.