People are angry that their economic systems arent compatible with having families either with current salaries or time committment careers require. Most people (women in primis) find themselves sacrificing basic quality of life to have 1 or 2 children. Government solution to such societal issues is to import workers in mass ("they will pay social security contributions for the elderly") which suppress wages down further increasing the cost-oppurtunity of having children. Replacement theories fit nicely, at least on an economic point of view. Businesses interests get cheap workers on one hand and childless consumers with more disposable income on the other hand. Entire societies dying is just an aftereffect

Short-termism of our leaders will lead to a systemic collapse just like in the Soviet Union.

Immigration is fine but Canada and the US are actively taking steps to dissuade the majority of their population from producing offspring in favor of taking in more immigrants. That's what people are upset over - not that immigration is part of the equation.