My biggest issue with the Divine Beast Dungeons were that all the shrines were mini tutorials for them, but it felt like they were too scared to make the puzzles more complex in fear for the player getting stuck. So the dungeons only changed the scale of the puzzles not the difficulty.
Also, I disliked how segmented they were. The best Zelda dungeons had one central puzzle which you continually found new ways of solving by approaching it from different angles - something like the central column of Lakebed Temple, the flipping floors of the GOAT Stone Temple, the timeshift stones of the Mining Facility. In BotW, dungeons are just... a collection of rooms. The puzzles aren't interwoven at all, they're just a few entirely separate (quite easy) puzzles that happen to be relatively near one another. Quite disappointing.
I do think there's a valid point that the typical 9 major dungeons of the series isn't as good a fit as it used to be. In design terms, that's an enormous amount of effort that would pull from the quality of the open world rather than just the shrines, and now that the re-realization of Zelda as fundamentally about exploration has been reached, that would be a shame. Additionally, I get the point that you don't want to break the flow of the world too much.
But I think the sweet spot on this had actually already been reached in a past Zelda title: Majora's Mask, which had the most challenging overworld in terms of traversal of any past 3D Zelda title and also had a strong focus on something not juts dungeon-y that the dungeons would break the flow of if overused. Majora's Mask has 3
outstanding dungeons, all contenders for the best in the series, that are moderately longer than the series norm and feature difficult, engaging, and interesting boses. It also has Woodfall Temple, which is still good for an introductory dungeon but obviously struggles given OoT had the Deku Tree - but this is an aside.
I'd like to see BotW go for that. They already realized 4 was probably the sweet spot in terms of the number of the Divine Beasts, but they didn't realize (or ran out of time) on the depth of design of the Divine Beasts. The best thing about Majora's Mask is that it actually has really complex dungeons - in terms of your ability, they're probably the most demanding in the 3D titles. Why? Because Majora's Mask tests all your skills in the overworld. You have to be reasonably good with the Goron Mask just to access Snowhead. That means Snowhead itself can just go all out. By contrast, the first 50% of all TP's dungeon content is a tutorial for the last 50%, which can be a bit grating.
Because BotW tests you extensively in the overworld, the Divine Beasts could really have gone all out, secure in the knowledge that you could handle them.