IG needs to invest into making Sentinels cheaper, common, replaceable, rugged. Put more men into more sentinels. Make the parts easy to make and replace. Make it allowable for substitute parts. On the table top, the sentinel is very bad, but from a realistic point of view, an all terrain vehicle that can be used inside large buildings, ships, city scape, forests is invaluable to draw light arms fire and breach a bunker or line is priceless. Sure a marine can do it. But they need to be deployed from orbit from a barge from another sector lifht years away thru the warp... If there were more Sentinels readily available, easy to maintain, and overall cheaper to build than a sophisticated power armor then a guardsmen can hop into a Sentinel when the sirens are blaring for immediate xeno attack.
https://youtu.be/Mw6CpFQ3Ctk