Conservationists are not concerned about introduced species out of some raw prejudice against "foreigners". They are concerned because so many of the species introduced by man have ended up threatening the very existence of native species, either directly by predation or indirectly by competition for habitat, food, etc. This is now a global problem as species, ranging from flat worms in Britain to cane toads in Australia, expand rapidly at the expense of native species, sometimes undermining entire ecosystems.
With a total world population of only 10,000-15,000 individuals widely scattered from Spain to Kazakhstan (it's main home), the white-headed duck was already endangered by human alterations to its habitat, and the last thing it needed was the human-assisted arrival of the ruddy duck which, with some 600,000 individuals in its American home-range, is hardly at risk as a species.
It may be, as Dr Ryder may have been trying to say, that since their inter-breeding produces fertile hybrids, the white-headed and ruddy should not be classified as two species at all, but merely as hitherto distinct races of a single species, and that therefore the hybridisation represents no significant loss of biodiversity. However, if, as is likely, the white- headed has developed special characteristics to cope with its habitat range, some of those characteristics might not be present in the hybrids. And that might only become obvious after a few years, when a largely hybridised population had been devastated by, for example, a periodic climate blip which the pure population could have survived relatively unscathed.