But Kelvin was an eminent scientist and everybody listened carefully to what he said - in 1900 he said that the Earth is 20 million years old and that was something you do not challenge.
Evolutionary biologists did object to such a young Earth as this did not allow enough time for species to evolve etc.
Rutherford was the first scientist to apply it for determination of age of geological samples.
Systemic errors were serious and the science of radioactivity was not understood well at that time. This certainly gave the established geologists a big excuse to criticize the new science and it was really the solo fight by Arthur Holmes that eventually brought radio-isotope dating technique to universal acceptance by about 1950.
Astronomers were discovering that the Universe is expanding and theorizing that the Universe might have started from a small compact volume less than 1.5 billion years ago.