This isn’t really correct. We need to chew food to make it more digestible to the stomach. If we swallowed our food whole in ‘chunks’, the digestive system would struggle. There’s no significant disadvantage to liquid food - we eat soup with no problems, and elderly people or those with some health problems find liquid diets much easier to digest.
There has been some discussion around the need to chew for optimal health and there is some scientific biological basis behind this argument (chewing releases certain enzymes within the saliva, and also signals the brain that you are beginning the digestive process), but it’s not essential (the brain / system is also signalled by taste), but if you’re concerned, Peppersmith do a sugar-free xylitol-only gum which can be chewed immediately after drinking Huel. Also it’s worth noting that Huel isn’t completely ‘liquid’, there’s still plenty for the stomach to break-down once it gets there. The food is just broken-down into very small particles, much as it would be if you chewed really well.
The MAIN function of chewing is to make it possible to swallow the food, and to increase the surface area of the food particles (ie make them as small as possible) to enable the stomach enzymes to digest the particles as efficiently as possible. The smaller the particles, the more nutrients can be absorbed. So from a human biology perspective, drinking your food in this way is actually beneficial to the digestive system and improves nutrient absorption.
Again a question for @JamesCollier the expert, but actually you can store uncooked rice and oats for a very long time as long as it’s stored in an airtight container, kept cool, and away from damp.
If you cook it or add water, yes it will go rancid very quickly indeed.