From what I’ve learned about him, I don’t think anything that Miyagi Sensei did was out of ignorance, or without reason. There’s a very interesting interview with Miyazato Sensei which has recently been translated where he speaks at length about Miyagi Sensei, and the lengths he would go to in order to further his training and understanding.
The attention to detail in Miyagi Sensei’s junbi undo routine is phenominal and a masterpiece in its own right.
Also its worth considering that Miyagi Sensei was extremely wealthy, apparently very well educated, and was in the enviable position of being able to devote his full attention to the development of his karatedo because he did not have to work.
I’m confident that Miyagi Sensei drew massively from other sources (Gokenki, visits to China, books which he was researching) and tried to mould some of these ideas around the Sanchin framework with a view of providing balance or more depth to his art.
Sanchin I see as a power generating exercise, whether you want to view it as a matter of attaining the best possible body structure, or more esoteric ki gathering/circulation. Whether we believe it or not, the whole ki idea was something which was very important to people and influenced much of what they did around the time that our arts were in their developmental stages.
Tensho I think takes these ideas further and starts building on some of the ideas already contained within Sanchin.