1) The ratio of people with a tertiary degree is probably too high. In past decades, only about the top 25% had university degrees:
https://www.cedefop.europa.eu/files/images/graph_month-stat_...
2) Fertility is heavily influenced by the availability of housing. Without immigration, house prices will fall and wages will rise, providing a boost to fertility until the population equalises.
https://www.nber.org/digest/feb12/impact-real-estate-market-...
With mass immigration you shortcut that, and simply end up replacing the original population, probably with negative overall effects if the incoming population is not genetically (average IQ, predisposition to violence) or culturally (respect for women etc.) aligned.