I think the reason the disappointment is there is the way the last three playoff "runs" ended for the Cubs. 2017, 8 total runs scored in 5 mostly blowout NLCS games against the Dodgers, 25 runs total in 10 games for the entire postseason(which is skewed by having 9 runs scored in the NLDS game 5 clincher). 2018, 1 run scored in the Wild Card loss. 2020, 1 total run scored in the two games that they lost. That is 27 total runs scored against 57 runs given up in 12 games for an average score of 4.75 - 2.25 per game. In the latter two seasons they had good records, but were barely existent in the playoffs. For a team with a lot of supposed offensive firepower in Bryant, Rizzo, Schwarber, and Baez, they didn't score many runs in the playoffs.
If they had won at least one more NLCS, even if they lost the series in the extra appearance, I think people would look back and say it wasn't a disappointment. Yes, everyone wishes to win a title every year, but most of us know that isn't going to happen. It is just too bad that the Dodgers managed to get their crap together in the playoffs right at the same time that the Cubs did. Although I don't think the 2017 Cubs beat the Astros in the series.
But if you think about it, after the 1945 World Series loss, the Cubs only won 9 postseason games total before the Joe Madden run began. Since 2015 they have won 19 postseason games, albeit with more opportunities since there was no permanent NLCS before 1969, no permanent NLDS before 1995, and no NLWC until 2012. So that run is epic considering the team's history. Under the old school rules, the Cubs would have made the World Series in 1984, 1989, 2008, and 2016 as they had the best record in the NL those years, meaning this run would have only ended up in the postseason once.