Thomas did see Jesus, and he believed as a result. How does it follow that Jesus wouldn't give us anything like the Shroud of Turin to see or that we shouldn't believe or be strengthened in our faith as a result of such things? Thomas was given something to see. That wasn't the problem. Nor was Thomas' desire for evidence. Rather, the problem was his irrational rejection of the evidence he already had, which was more than sufficient (fulfilled prophecy, Jesus' other miracles, his prediction of his resurrection, etc.). John 20 isn't opposing a desire for evidence. It's not opposing Jesus' visibility in the world, whether through the body Thomas saw or something like the Shroud. Rather, John 20 is opposing the sort of irrationality that mishandles evidence that's already been provided. Thomas' problem wasn't that he was too rational or too concerned about evidence. His problem was that he was irrational and mishandling the evidence he already had.
The people John was writing to wouldn't see the risen Jesus the way Thomas did. But they would have a lot of evidence, such as the document John was writing to them. That's why John is so careful to cite evidence along the way, such as prophecies Jesus fulfilled and the eyewitness nature of his (John's) testimony. John incorporates some of the historiographical standards of his day in his gospel - he writes in the genre of Greco-Roman biography, he appeals to eyewitness testimony, he appeals to witnesses who were present "from the beginning" (a significant phrase in ancient historiography), etc. - because he was concerned about evidence and wanted his readers to be.
Much more could be added to my comments above. For example, the apostles performed miracles to verify their authority claims. People would often see evidence of those miracles (a crippled man walking, a blind man gaining sight, etc.). For these and other reasons, it doesn't make sense to read John 20:29 as some sort of condemnation of any belief based on sight or a condemnation of any desire for evidence, for example. Faith isn't the same as sight, and it doesn't always result from sight. But it often does involve sight, as well as other forms of evidence. God can convert people or strengthen their faith independent of historical arguments and other evidential means, but he also provides us with evidence and often uses that evidence to work in people's lives.