Such big companies and the services that only such big companies can provide have also enabled the development of many new businesses. Its usually those who are complacent and could not adapt to the new business realities who usually get destroyed.
What services would these be? :)
Honestly, I think for a capitalist model what Adam Smith envisioned would be better: a bunch of small companies competing with each other. What small benefits the bigger companies do bring are offset by the problems they bring [1].
But, even Barnes & Noble can't stock all the books that Amazon makes available. With Amazon, if I need, I can get it the next day.
I have a very small company that I run off my regular job hours. I wanted to do some marketing, get some paper, some other supplies, print stuff in the middle of the night and post some flyers around town. If Walmart weren't open and didn't have the supplies, I would have been limited to what I could work out during the regular hours of the small businesses there. The horror is, I don't know why, many of them simply shut off at 5, books shops and some others. Thats crazy.
There was this printing I used to get done at a local business, to be honest they were incredibly sweet to deal with. But their lead time was 3 days at best for printing some 1500 flyers. With OfficeMax in the same city, 2-3 hours, sometimes 30 minutes. It gave me incredible flexibility to change things on the fly - typos, content or whatever.
These were the real situations I faced. Without them, my business might not have been impossible, but definitely quite a bit more complicated.
And, of course, everything has its pros and cons.