Grand Kahuna battles are just battles without a trial or puzzle attached. In that way, they are absolutely weaker than Gyms that actually come with their own location and puzzle with Trainers. You can count them as Trials if you want, but I feel they're distinct enough that most people don't consider them Trials--rather, they're more like fighting weaker versions of the Elite Four throughout the game so you can rechallenge their stronger teams at the summit of the Pokémon League. As such, SM actually have less Trials than Gyms of past--there's a Fairy Captain with no Trial, and a Dragon Trial with no Captain, which felt very unfinished and unpolished! Totem battles were okay, but it shows just how difficult it is to balance the series's battle system with that focus on Types--you literally need inherent stat-boosts and 2-on-1 disadvantaged battles to pose any sort of challenge, and even then most of them ended up quite easy if you, as always, had the right Type advantage. Challenge in Gym battles can be improved by including a new gimmick, inherent stat-boosts, or even like the above idea, not making Gyms focus on a single Type (but only after we get our Dark-type Gym, please!).
I don't remember exactly what the Trial pieces allowed you to do than "just 'permission to use Cut.'" All that comes to mind is once you completed certain Trials, the guides would remove those ugly roadblocks so you could explore more. But roadblocks break immersion and most people seem to dislike their overabundance. On the other hand, gen 5 badges only affect the level that traded Pokémon obey you at--you can use HMs whenever you receive them, which is really fun because if you explore around you can get HM Waterfall quite early and gain access to lots of fun little areas as early as around the 5th Gym! While I hope they take a more Let's Go-inspired approach to their HMs this time around, it shows that even HMs don't have to be tied to Badges.
Trials were certainly an attempt to breathe new life into the concept of Gyms, but in the end were nothing more than Gyms with a tropical coat of paint. Anything that could be considered an improvement from the Trial system can be easily incorporated back into the Gym system, which is why I see no reason to worry over their return.
