I would strongly, strongly disagree with this assessment, with all due respect.
Regarding the concept that the Arabs are tied to the nearby regions of the Arabian peninsula, this is an anachronistic opinion. In the times past, the Umayyad was not simply an Arab state, but an Islamic Caliphate, whose role was clear-cut, both in their words and their commentators, to spread the Sharia and thus Islam, to the entire world. The Umayyad war machine pushed to the far east and far west in this endeavor that we refer to as jihad and their armies existed across both Asia, Africa and Europe. There were Arab soldiery/muhjahideen on all the fringes of the Islamic world, pushing the frontiers in all directions and or defending the borders. It is not as if the Umayyad lacked the will to maintain the war in Hindustan or such, but that the Chalyuka and Rajput had a period of strength combined with difficult situations in the Umayyad realm, predominantly the frequency of rebellion and the then more pressing matters in Central Asia that posed greater threats than the Indian states. The Umayyad had been seeking to match the enemies within Central Asia, especially the Tang and Tibetans and the regions of Bactria, Sogdia, Ferghana and Zabul, remained outside Umayyad control. The Kwarezm itself was only minor Umayyad control and recently in the middle Umayyad, the Umayyad had suffered some defeats to the Tang-Karluks and had competed with the Tibetans in the Kashmir (with Umayyad general victory in the area and also an eventual conquest of the norther Buddhist Shahi states in Afghanistan). In other words, the Umayyad were competing incredibly on all fronts already, failing to conquer the most populous region on the planet while you are maintaining wars on 6-7 different fronts and across massive distances that exceed the length of any empire in the world at the time (including the Tang), is not a large failure.
The Umayyad campaigns also seemed to have had a large assortment of troops and naval companionship. In other words, there was enough soldiery to maintain sustained wars in the region of Hindustan, this is given evidence by the relative success of the campaigns, which at least battered the Hindu states. Meanwhile, the Umayyad are unharmed in anyway by these Hindu states, which are on the constant defensive, while the Umayyad have essentially all the time that they wish to launch invasions and with each invasion, the rate of success increases, as the Islamic armies as I mention, likely posses more soldiery and also have much less economic mal-effect from the wars. With all of this warring in India, it should be noted that within the Umayyad already, there is a slow creeping turmoil due to internal revolt, especially fromt he Khawarij, who sapped the Umayyad of many recruits and also hampered the extent to which armies could be raised. Such as the famous case of the Peacock Army, ordered by al-Hajjaj ibn Yusf to invade the Zabul and establish a beginning conquest of the Northern sections of Hindustan through the Khyber Pass, was infiltrated by Khawarij using 'Kitman' and rebelled int he field and turned on the Caliphate with an army of 50k warriors. Further, the region of southern Balochistan, had become a hotbed for Khawarij banditry and defectors/turncoats who fled the fronts of the wars in the east and joined varied bandits and rebel Imams (of the Khawarij variety), this situation in the Baloch region would not change until the middle Abbasid period, when the Ya'qub ibn Layth al-Saffar defeated the Khawarij Imam Yasr ibn al-Yammar in around 858 (I do not remember exact date right now). In conclusion, the Umayyad did very well for its circumstances.