I have massive regards for Frenkie' s talent and have been one of his strongest advocates here. But to simply pack Cassado aside as inferior is myopic. They have their different strengths. In my view, Barca is very lucky with current midfielders( if you add Bernal)
For matches you need control and coordination from the base of your midfield and need to gradually build up the attack using spaces, Frenkie is a perfect fit. This will be against teams that play low blocks or use man marking methods. But be aware that you will lose a lot of interception and combating tackles to break up counters and maybe some speed/acceleration in the attack which may reduce some sort of pull through passes that a player like Cassado offers.
For open matches that require a gung-ho back-to- back/gengerpressing approach, especially when the opponents come to play too, Cassado is a much better fit. His combative tackles, interceptions and ability to break up attack is still the best in the team(bar injured Bernal). He also comes with a vision for those penetrative passes. You might lose much in control and build up play though.
I have been a keen advocate for FDJ while not regarding Cassado initially. However with more matches I saw what Cassado brought to the team which was one of the reasons behind the team's flying start early on in the season, completely assimilating seamlessly into the new playing style introduced by Flick.
They are both good and have their different uses. The good thing about the current Barca side is, Flick, who is not only experienced enough to use players to their strengths but also very flexible in playing style(unlike the previous fraud).
Enjoy both rather than compare.