What are some arguments against reform in the late 1800s?
I have read some of Sumner's writings and understand that his view was that government intervention went against "Nature." Were there others who spoke out against reform at that time and what were their reasons?
The Gilded Age economy resulted in stunning economic success and wealth by the upper class and the industrialists, creating wealth so obscene it's almost impossible for us to comprehend it in modern terms.
Of course, those in the upper class sought a philosophy that would justify amassing such riches when so many others were poor. Charles Darwin's 1859 book, On the Origin of the Species, put the evolution of mankind as the result of competition, whereby the weaker species and members of a species did not survive, resulting in a stronger gene pool. While he was meaning the human species and not economics, robber barons of the age grabbed on to the "survival of the fittest" mantra and argued that the wealthy were supposed to be wealthy because mentally and physically, they were the strongest.
Anthropologist Herbert Spencer subscribed to that belief, as did mega-wealthy industrialist Andrew Carnegie, as expressed in his The Gospel of Wealth. (He had built his entire fortune on his own, rising from a lowly office clerk to owner of an empire of steel) They both believed that large scale reform to business or society would interrupt what was essentially a process of economic natural selection.