The fear of a slippery slope was mentioned twice, first when the euthanasia law was accepted in 2002, and it never became a reality. The numbers have never gone up.
The second time was with neonatal euthanasia. Some people expected huge numbers of newborns to be killed, but it never happened. Instead of three cases every year before the legislation, we went down to three cases in 15 years, so there was no slippery slope there.