No brainer

I like the brains too. Even if not detachable. It’s nice to have at least one decent-size zippered accessible pouch. Not that hard to MacGyver one to a roll top.

https://seekoutside.com/backpack-top-lid/

I got the Zpacks Multipack, but these days I just put it inside the roll top if I have the room.
https://zpacks.com/products/multi-pack

There’s always a work-around – been using brains on and off since 1975. Pays to be flexible.

Have both pack types.  What I usually keep in the brain, I store in a ziploc when using the roll top, and pack at the top.  This way my organization system is consistent and I’m not hunting for something and it gives the benefits of both types.  I much prefer the roll top.

I have an old Flash 45 with a brain.  Recently used it after a long hiatus on a weekend trip.  I do miss the convenience of being able to quickly stash and pull out things that I might need during the day.  That said, the roll top isn’t too difficult.  I think I’ll start putting my needed items in a separate bag as David D suggests and placing it on top.