Typically in a metroidvania you’ll be using the new abilities you’ve unlocked to access new areas, each one being something of a puzzle or introducing new mechanics to the player, so it’s rarely just a walking simulator.

I find gaining a new ability/unlocking new area to be the rare event that happens every hour or two; whereas, the mundane backtracking or traveling from one area to another is basically most of the gameplay.

I'm not implying they're walking simulators; however, overall they have poor gameplay density compared to an action game or a shmup.

even in something like a shmup there’s enemy and projectile patterns that are so trivial they’re essentially inconsequential.

I would not call frequent downtime in a shmup good design, TBH.

Maybe you're referring to the few seconds before every boss/miniboss, which is nowhere as frequent or lengthy as traversal in metroidvanias.

Even in shmups predominantly designed for score chasing like Devil Blade Reboot, the challenge is not dodging bullets—it's point blanking every enemy to get the highest score possible, so a trivial bullet pattern here doesn't impact the player's level of engagement or gameplay density.

Interesting point though, hopefully I don’t see what you mean when I next play!

I hope you don't! Low/high gameplay density is not necessarily good/bad, it depends on the player's preference and expectations. If you enjoy the downtime, have fun!