I've played competitive pokemon on and off since Gen 4 - and never once have I dealt with something as unenjoyable as what's going on right now in the metagame. I've never felt the need to post or participate in the suspect process not once (even when Mega Kangaskhan was running around) - until now. As a matter of fact, my last non-recent post is from a Gen 4 RMT from 2008. My very next post is from a few weeks ago bringing up how awful dynamax is. That's a whopping 11-year gap.
This mechanic is completely absurd and has easily made this the worst laddering experience I've ever had (which is impressive since I thought nothing would beat Mega Kangaskahn). It's turned the metagame into a total joke and infested every single game that is played with its awfulness. There is no escaping this mechanic that almost always has a dramatic impact on every match.
Because a lot of points have already been mentioned, one thing I'd like to bring up is the risk/reward of regular play and how dynamax completely skews it beyond belief.
That's because in the vast majority of situations the person who Dynamaxed first has the advantage. Once they've dynamaxed they've likely either:
1) Taken out or severely weakened a key pokemon on your team
2) Accumulated stat boosts to set up for a sweep (often getting multiple boosts while simultaneously dishing out huge damage)
3) Doubled their HP to render your check useless (see Gyarados surviving thunderbolts)
4) All of the above, at the same time
Meanwhile, you have none of the above. Dynamaxing in response to their dynamax is almost never ideal - by the time you do it they already have huge momentum while you're scrambling to do damage control. And the unpredictable nature of dynamax just makes this all the worse.
In most cases the only way to safely counter someone else's dynamax is to do it on the same turn as them - which is not at all easy to do when they can literally do it on any individual turn in the game. It puts huge pressure on you to make the exact right read at the right time - and if you're wrong it's probably GG.
It's as if GF went out of their way to make defensive dynamax a much shittier option. If you want to defensively dynamax to try to survive their onslaught, all of your non-attacking moves turn into Max Guard. Things might have been interesting if we got a Max Paralysis/Toxic/Sleep/Restore to counteract their sweep attempt, but we get none of those. If Ferrothorn wants to dynamax to survive against Gyarados, he actually LOSES all his options and can only Max Guard while Gyarados nukes him and gets his boosts on the other turns. (Seriously, why the hell do we get 2-3 Max Guards in our movepool when we can't even use them consecutively?)
Before, setting up to sweep or using a choice item represented commitment and opportunity cost. You're either trading immediate damage and risking them staying in and damaging/statusing you with the former, or trading the freedom to choose between your moves with the latter. There are clear downsides to each that you have to work around by planning and playing intelligently.
That's the big thing here - you have to be intelligent with how you do things. When planning to sweep you have to take into account what they have left and what the situation will be like a few turns down the road. When using choice items you have to predict what they're going to do and risk losing all momentum or even getting crippled or set up on if you choose incorrectly.
Dynamax removes all of this from the equation. You can happily set up and ignore all of this as you get massively rewarded for doing so. Your double HP means your check can't kill you, the increased power means you can now out muscle them, and the added boosts mean the effects last long after the three turns are up. If the risk/reward in the past was 50/50, dynamax now turns it into a 90/10.
Right now, people are awarded for setting up or firing off choice moves mindlessly. It doesn't really matter if they're wrong or what happens next, because dynamax's doubled HP boost and additional effects override all of the normal repercussions.
Things like Gyarados don't have to find the best time to setup. They don't have to think about what move to use. They don't have to consider what can KO back in return. They'll just fire off nukes that boost their stats while having the HP to survive damn near anything that would normally kill them. And if they don't end the game outright they'll likely leave a giant, gaping hole in your team that's incredibly difficult to recover from. And while this isn't always the case, it is the case the majority of the time.
Choice users like Darmanitan or Dragapult don't have to think about what move to use and the risk they're taking on. If they pick wrong, they can erase the commitment of their choice item, seriously threaten or kill your wall, and at the very least force you into a 50/50 that didn't exist before.
While I didn't play much of Gen 7, it seems that Z-moves were a contested topic. But this is so much worse it's laughable. Before you had ONE power move that you got to use, ONE time in the match, at the cost of your item. You don't get the benefit of having a normal item and if you use your Z-move incorrectly, you essentially just wasted your item slot.
Imagine that, but instead you get to choose between three Z-moves. These moves have similar power, but unlike Z-moves they actually boost your stats or set up weather to increase the power further. Even choosing the wrong coverage move is a benefit as your stats get boosted. The effects of these boosts last long after the 3 turns are up (often being enough to end the game). Not only that, while you use these moves your HP is doubled. And, you get the benefit of using your item the entire time. It's not even comparable.
Lastly, I want to bring up the alternative of NOT banning dynamax. If this is the case then we're inevitably going to have multiple suspects for the most broken abusers of this mechanic. The question is, how many do we have to ban in order to have a somewhat balanced metagame? 5? 10? More? How much time would the suspects for those take? What happens when a new abuser pops up and now we have to go through the suspect process for them as well? What happens to UU and below that now doesn't have Ditto to protect them from their respective dynamax threats? How many threats need to banned in those tiers? What if the problem is even worse than what we're seeing right now? And what if we need to go back and just ban dynamax after realizing the metagame isn't salvageable? It's not a good look.
This metagame is already a circus, but choosing not to ban dynamax will make it a completely awful experience for a lot of people. It's going to be the same as a toilet that's clogged up and overflowing onto the bathroom floor - a shitty situation that just gets worse and worse.
TLDR; ban this shit and never look back.