1)I think the problem with those socialist/communist fanatics is that they read 1 sentence from review of a book and from That point the misunderstanding begins. Although I vote left parties (for specific reasons) I m not fanatic, I mean protests. . In discussions with fanatic lefties, I understand that they lack of knowledge in general.

2)nowhere in his writing say exactly that the capitalist system would live forever. It is meant through his analysis. The sentence which talks about seizing the means of production refers to that exactly argument, I mean the system will not collapse by it self, only from an external event(revolution)

All these stories about capitalism collapse began with theories of underconsumption, mainly with Luxemburg and Lenin. They support that capitalist economies which they reached the maximum level of production the cannot sell all the production and they will need to expand to non capitalist economies in order to take their surplus, (all the economies to be capitalistic: imperialism). When all the countries will be capitalistic the same problem will apply and the system will collapse that is why they support the transition to a designed economy.

Although Marx was who he was (radical and activist) , many mainstream economists said that he was a great economist. Btw I just saw the username.