invisible hand of market is myth and dogma produced by corporates economs to prevent system to create regulations wich can help small business and reduce power of corporates
so don't expect anything from that hand
a 51 attack will be soon solved by bijective function
There are valid reasons why costumers favour big businesses over small ones (lower prices, reliability), and other valid reasons why some customer prefer to deal with small business (contributing to the local economy). There's no problem with big corporation having more power if they do indeed provide more value, and that's something that the free market is well adapted in judging that.
There's no need for regulations.