Billionaires in Germany do not openly donate to the AfD for fear of jeopardizing their businesses through potential consumer boycotts. Alois Müller is a good example of this.
You can still assume, however, that enough money is flowing into the party’s slush funds via front men, bypassing the officially reported party donations.
This phenomenon can be observed in all far-right parties. The American Republicans are an exception in this regard because they are not a new phenomenon, and the influence exerted by billionaires has been de facto legalized since the Citizens United Act.
It is not capitalism itself, but rather representative systems of government that sooner or later lead to corrupt oligarchies.
Even socialist states have corrupt elites, so it doesn’t depend solely on the system of government.
But yes, with regard to our situation: Democratic capitalism is coming to an end, because democratic means cannot be used to exploit the population any faster or more intensely (though in a system based on infinite growth, this is a necessity)—which is why things are now becoming undemocratic once again.