Funny that you mentioned Flask, which literally started as an April Fool's joke [0], but that hasn't stopped it from becoming the 10th [1] or 11th [2] most popular web framework, with over double the number of devs compared to Rails, so "seriousness" likely has nothing to do with making people want to use it.

[0] https://lucumr.pocoo.org/2010/4/3/april-1st-post-mortem/

[1] https://www.statista.com/statistics/1124699/worldwide-develo...

[2] https://survey.stackoverflow.co/2025/technology#most-popular...