I think there are 2 separate issues that you might be conflating here. Just because one believes that the Bible predicts a future age where the sins of man wax greatly doesn’t mean that you believe the Gospel will fail in this life or this world. The triumph of Christ can be seen as already completed on Calvary, and in fact inevitable since before man was cast out of Eden, and nothing that happens or will happen in this world can change that.
You have a positive outlook on the role of Christians and what we can accomplish, and I agree that is good and something we should encourage. However, I think it’s also good to heed the warnings in the Bible that many will claim to follow Christ and do His work, but He will not know them. Just looking at the number of self-professed “Christians” today doesn’t tell us much about the state of the church and its work.
When a Christian says life will only get worse so what's the point in working towards and expecting a positive Christian community, cui bono?