Ultimately, the answer must lie in the distinction between the general, and the specific and personal. Take two contrasted examples.
If you believe that the political system in your country is broken beyond repair, then there are two things you can do when elections come round One is not to vote. If a large enough group of people refuse to vote, the system itself loses credibility. In several countries now, barely 50% of the population now vote even in the most important elections. A political system where less than 40% of the people voted would simply not be viable and would have to be replaced. The other thing you can do is to vote deliberately for parties that are going to crash the system more quickly than would otherwise be the case. They may be incompetent, they may be extremist, they may be completely out of their heads, but votes for them take away votes from the established parties, and so hasten the end of a system which is anyway doomed to disappear.
On the other hand, and in spite of Brecht’s thesis, we shouldn’t neglect our human duties. Giving food to the hungry or money to beggars doesn’t really perpetuate a system. Not doing so is often just a pretext to justify selfishness. There’s a big difference, in other words, between things we can directly influence, and those where our influence is only indirect.
But there are objections to this thesis aren’t there? Won’t it just make things worse?