I have a proposition that might help balance out the Level of power Dynamax can bring to the table. I want to start off by saying though that i think outright banning Dynamax at this time is a tad rash. With every new gimmick there will be abusers of said gimmick, and assuming Game freak doesn't bring Mega's and Z-moves in later series, we can assume that Dynamax has a 2-3 year shelf life (assuming gen 9 comes out then). I, for one, am against outright banning a mechanic this early into its life without exploring more options of counter play, game play interactions, and movesets. And in regards to Gigantimax, most of those mons are weaker than regular Dynamax counter parts.
Now to my proposition. Clearly mons like Gyarados, Excadrill, and Hawlucha are prime abusers of Dynamax that can snowball. The biggest complaint i'm seeing isn't so much these mons as it is the potential for these mons to be the Dynamax user. Slap 3 of them on the same team and have fun guessing, if it even is or will be them to Dynamax. So what if instead of banning the mechanic, we augment when it can be used?
In prior gens, Mega evolved pokemon were known because only a select few could Mega Evolve. In battle spot it was common to bring multiple mega users and make your opponent guess which mega they would bring/use. For Z- moves it was a once use nuke that anything could use, but since it was a super move, you would almost never see defensive mons like Toxapex utlize them in such a way.
Due to Dynamax being as free of use like Z-moves and not having the exclusivity of mega stones (not counting Gigantimax forms), I propose something that will make Gigantimax slightly better than Dynamax in when its used. What if we regulate the use of Dynamax as a comeback mechanic, only to be accessible by the last remaining party member? Meaning, in order to access the mechanic, 5 of your 6 pokemon need to be ko'd. Let's face it, unless taking another Dynamax user head on, there are few chances currently that having a defensive pokemon Dynamax is better than an offensive one. This would preserve the mechanic, while also promoting smarter play with when you switch out your pokemon.
Here is an example scenario. You have remaining a U-turn/Volt switch user on the field and a scarfed Jolteon as your only other remaing team member. The opponent has a Pokemon on the field that you outspeed and a Hawlucha as their last. When the oppoents pokemon goes down, Hawlucha is free to Dynamax, however you can't since you have 2 mons left. However, if you ko with U-turn and pivot into Scarf Jolteon, you will have the type advantage and potential speed advantage, but not the Dynamax advantage. The opponent has the mon deficit and type disadvantage, but they have Dynamax as a potential crutch to make a comeback. Should Hawlucha succeed in defeating Jolteon, now you can Dynamax.
Now for Gigantimax. As far as i'm concerned, these should be viewed the same as Mega's, powerful yet predictable. Like mega's, some are simply better than others. Dynamax Coalossal can set up sand with its rock max move while G-Max version sets up a rock version of fire spin. Clearly one is inferior to the other. That said, both Dynamax and Gmax are possible in game. So to resolve this issue, let's actually treat Gigantimax pokemon like mega's under certain condtions. Much like when choosing if a Pokemon is shiny, we allow the mons that can Gigantimax the ability to become those forms by choice - they are either Dynamax or Giganatimax. Assuming the first idea i had about Dynamax being a last stand mechanic became a reality, let's make Gigantimax forms be the exception to the rule. Pokemon that can Gigantimax can do so at any time during a battle, however they must become their Gigantimax form. This would be akin to having multiple pokemon in your party that can mega evolve, but only one can. You don't know if it's coming, but you can spot the threats that can. In turn, this would mean that should a mon Gigantimax, the problem children like Hawlucha and Gyarados can't. It gives players the flexibility to change a match on the fly, while limiting what can snowball.
In short, this idea could preserve Dynamax while creating an interesting meta, while mitigating set up pokemon from snowballing after a boost or 2 midgame. Weather setters would still need to fufuil their roles, sweepers would still be threating, and defensive dynamax to buy time for chip damage and/or boosting moves would be better.
