The psychatrist doesn't seem to know much about schizophrenia. If you compare worldwide prevalence [1], you'll see that the difference between countries of lowest prevalence and countries of highest prevalence only differ by about a factor of two. If you only compare developed countries, the difference is less than 50%. So even if everyone in Germany would start daily consumption of high THC cannabis, and IF that would be the only cause of schizophrenia besides a societal baseline, there would be about 8000 new cases per year. If we assume that each case costs about 16k EUR per year [2] for health care, that would result in additional costs of only about 130M EUR per year. That's .003% of GDP, so completely insignificant.
But what's way more important: The study shows that the only correlation is with daily use of high THC consumption. No correlation with low THC products (see figure 2 in the study). Results from e.g. the alcohol prohibition and recent cannabis legalization in the US suggest the share of high THC product would actually decrease once cannabis would be legal in Germany. So IF high-THC cannabis is the main cause of schizophrenia, it would actually be probable that legalization would decrease schizophrenia incidence in Germany.
Going into psychosis you aren't working either. And schizophrenia will decrease your lifetime working hours. So you need to add those too. And you'll need $$ for disability (beside healthcare) to also help your children. And housing etc. (not everyone, but many)
But yes, if the number of people is low it won't be a problem..
Actually many schizophrenia patients are in fact working and living relatively normal lives. The costs I linked are the actual current costs for Germany and include reimployment measures (have a look at the second link). But you're right, it is of course an outcome we as society should try to prevent anyway.
My point was that there won't be a societal crisis, even if the assumption of THC being the main cause of schizophrenia were true.
With or without legalization, better drug abuse prevention through education, widely accessible addiction treatment and de-stigmatisation of mental health issues are things we should strive for - because those measures actually affect peoples lives for the better.
But what's way more important: The study shows that the only correlation is with daily use of high THC consumption. No correlation with low THC products (see figure 2 in the study). Results from e.g. the alcohol prohibition and recent cannabis legalization in the US suggest the share of high THC product would actually decrease once cannabis would be legal in Germany. So IF high-THC cannabis is the main cause of schizophrenia, it would actually be probable that legalization would decrease schizophrenia incidence in Germany.
[1] https://en.wikipedia.org/wiki/Epidemiology_of_schizophrenia
[2] http://www.gbe-bund.de/gbe10/abrechnung.prc_abr_test_logon?p...