But there is a reason why even the biggest brands spend on promotion.
Promotion is an investment and every investment has risk attached. If you believe in the product you are selling, you have to risk it.
I am not trying to demean your books by calling them products, but when you sell in the open market for profit, then the reader is the consumer and your book is the product.
Even individuals are products, look at all the social media influencers, they are selling themselves as promotional products to brands!
Cheers and all the best with going the traditional publication way. Hope it works out!