Firstly, India or South India weren't whoile geographical units even though successive rulers tried to get them together as a whole. The British managed to cobble together one entity and broke it up when they left !
There were organized and urban communities around trading posts and then there were dwellers in and around forests, pastoral with innovations in agriculture as the demands for grain trading increased. The principal grain was rice which served as a staple food as well as a base for the heady brew!
Idli must started off as cooked broken rice and a watery preparation of dal plus red broken chillies. This must have been the preparation of a poor man or a woman, who wasn't allowed to hunt, or preferred to spend minimum time on food and pursued more thoughtful occupations.
Many households also use cooked rice saved overnight and steamed again and eaten with a mash of remnant chillies and coconut crumbs. Most Udipi restaurants continue this practice. You can find a lot of them around temples where coconuts are plenty!
The kings and the richer folks would have thought it below themselves to eat anything less than meat and sweets for breakfast ! So let's give all the not-invented-here theories a pass. Folks in India pursued non-meat diets for economic and spiritual reasons, I suspect in that order !
Another tip for idli lovers.When in south India, particularly Tamil Nadu and Karnataka, if you're really hungry at breakfast or dinnner, order idli first. It usually arrives in next to no time. After taking the edge off your hunger, you can then order that masala dosa you've been yearning for.
