A key to the loyalty of the Indian Army was that, notwithstanding the differing status of the British and Indian troops, they all fought together. Took the same risks, suffered the same conditions and were comrades in arms. If lead by British officers the officers were expected to lead from the front. If supported by British units those fought in the same battle and took the same risks.
If you keep the British working in home factories, earning good money, whilst the Indians go to the front it would not take long for the Indians to query why they are there. A good way to accelerate Indian independence and spur a mutiny. The Indian Army fought loyally with the British, not for the British. That was the approach of Sergeant Major Mohandas Karamchand Gandhi in his choice to serve the Empire. By serving alongside, Indians would earn respect and a Dominion type status in the Empire. If it, instead, became serving for the Empire as cheap cannon fodder, then it undermines the entire raison d'etre of the OTL Indian Army.
Mention has been made of Indians feeling insulted if they were not sent to fight. This was not unique to Indians. The same issues came up with other Empire armies composed of volunteers. They did not volunteer to heave cargo, maintain vehicles, sit in an office and a thousand and one necessary jobs to keep a modern army in the field. So the British Army did a somewhat over large proportion of these non combat tasks. This was one of several reasons for recruiting labour corps such as civilians from China.