Many products will likely never be available for same-day or 1-day shipping, I agree. And consumers will still see an increase in the number of products that ARE available same-day and 1-day which is an improvement any way you look at it for the consumer.
But, I'm talking about shipping costs.
Example:
Joe orders a shirt from shirts.com and has to pay $11.99 for 2-day shipping. If Joe can order the same shirt from shirt.com's Amazon store and pay $7.99 for 2-day shipping he'll likely do that.
If shirts.com can profit more from selling on Amazon than on their own website, how do you think they'll direct their traffic?
If you extrapolate this concept out to the thousands of e-commerce businesses that Amazon would benefit, we're dealing with a substantial loss of business.