Both Isaiah and Paul were looking for things to get better. Isaiah was still predicting a restoration of the nation of Israel. He expected things to turn around and for the descendants of David to return to their thrones, and in fact enlarge the reign to include the whole world. Isaiah saw the coming of a new heaven and a new earth. Isaiah sounds like John's Revelation from time to time, writing:
See, the LORD is coming with fire,Isaiah 66.
and his chariots are like a whirlwind;
he will bring down his anger with fury,
and his rebuke with flames of fire.
For with fire and with his sword
the LORD will execute judgment on all people,
and many will be those slain by the LORD.
Paul writes to the Philippians about another vision of life after.
I am torn between the two: I desire to depart and be with Christ, which is better by far; but it is more necessary for you that I remain in the body. Convinced of this, I know that I will remain, and I will continue with all of you for your progress and joy in the faith, so that through my being with you again your boasting in Christ Jesus will abound on account of me.Phil. 1. Notice that Paul is undeniably talking about a reward in the afterlife. As surely as Isaiah was talking about a return to worldly power of the Israelites. Similarly, he writes, "But our citizenship is in heaven. And we eagerly await a Savior from there, the Lord Jesus Christ, who, by the power that enables him to bring everything under his control, will transform our lowly bodies so that they will be like his glorious body." Phil. 3.
I think they are both right in that faith in God holds hope for all. I think they were both wrong about how it would be manifest.