As 1 Thess 4 shows, Paul expected the Coming to take place during his own lifetime. The Thessalonians were concerned that Paul had taught them to prepare for the imminent Coming, and they had written to him that some of those who had received that message had since died, which they could not understand. (When decades after, Paul's expectations were not fulfilled, another writer modified his epectations; we now call that pseudonymous writing 2 Thessalonians).
The people who wrote of "this generation" in their Gospel similarly expected the imminent Coming.
Their expectations have been repeated throughout subsequent generations and no doubt will continue to do so for the forseeable future. Learn the lesson that history teaches. (Of course, the story does help some people cope with the pressures and problems of life.)
Doug