I don't understand this. Not only is it useful for mining and breaking dungeon walls to make the run go faster, but the dragon fireball is fun to spam when i am bored. In fact I am going to argue that watching the dragon fireball dig a pit in the terrain is the most fun thing for me in this game.
is this breaking something in the game, or are the devs just taking it away because its "too fun". I mean the dragons have been in the game for months now, if there was a problem they would have been nerfed a long time ago. The devs said something like they think that players strafing the ground with the dragon seems too crazy in the stream and I don't think that is a valid reason to take away player enjoyment. They even said to enjoy it while we could implying that they know what they do is not fun. It reminds me of jay wilson in diablo 3 and their policy of nerfing "fun" things that don't hurt the game for no reason.
if they must take it away at least compensate for it by letting dragons shoot fireballs without having to be in flight otherwise trying to mine a node, or break a wall would require multiple recircles and then I might as well just lob a bomb or something which is honestly more effective anyway unless im mining outdoors due to the accuracy and the fact that I can actually use it inside a dungeon since most of the time dragons are too big to fly in the tunnels.
i've done u5 and u6 challenges every day 8 + hours a day for the past week or so and I only encountered the issue where people fireball a hole through the floor causing the boss to fall to another level maybe 1 in 10 dungeons. Even less if the zone has mostly underground dungeon because it is "literally" impossible to consistently fly in a tunnel so people cannot "literally" spam fireballs on top of the boss at least half of the time. I don't see a problem here, and fireballing the dungeon makes it easier to get to the boss did you take that into consideration?
If there are 10 people with dragons fireballing a boss that is on an upper level floor ON PURPOSE the boss is still going to fall through another level whether or not the rapid fire is nerfed. And if people wanted to they can achieve the same thing with bombs. In fact it only takes about 2-3 well placed bombs or firebreaths to knock a boss from an upper dungeon floor the a lower one, therefore your consideration is irrelevant here because the effect from the "spam" can be achieved just as easily without the spam.
the dragons firebreath is completely useless in underground dungeons and 80% of the time you can't even use it because tunnels are too cramped so spamming fireballs in a lower level dungeon doesn't happen. As with what most people do I spam bombs to get to the bottom dungeon floor after I destroy the top building with everyone else so I can actually see the portal without having to go though a maze where I cannot see anything because everything is too dark in certain biomes such as the dragon and dark fey. And since there are so many people at the challenge I never have to use more than 3-4 fireballs anyway before the building is broken sufficiently. This plus the fact that griefing can be achieved just as easily if not easier with a bomb because of the accuracy leads to the conclusion that your reason for the nerf which is because its "annoying" in challenge groups is incorrect.
The people this nerf would hurt the most during a challenge are people who arrive late during an assault on a castle dungeon, because they don't know where the breach in the wall is and they have to make their own or just sit and leech. and if the dragons rate of fire is nerfed then obviously it will require several recircles to break through the wall properly. By then the dungeon is either over, or they already missed out on 1 or 2 encounters. Clearly this hurts the "single or solo or late" players". The idea that dragon breath spam is annoying in groups is therefore irrelevant due to the reasons stated.
And so again, if you have many people with dragons then tiles will be destroyed no matter what, It doesnt even matter whether or not they nerf it. 10 fireballs is still 10 fireballs and 10 fireballs or a million fireballs on a ground floor dungeon doesn't do anything to cause problems because the boss isn't going to get lost somewhere on a different level. However 10 fireballs with a lengthy cooldown on an upper level dungeon will still cause problems if people are doing it on purpose no matter what. Not to mention a single bomb on an upper level will cause the same problem as a single fireball or 10 fireballs on an upper level dungeon floor boss so adding a cooldown for this reason is invalid.
My contention is that the nerf effects solo play and solo play is the only thing thats important in this discussion due to the above reasons which is basically your problem that you specifically stated will persist nerf or no nerf. dragons don't work in shadow towers which is the only group play that matters and so as such is irrelevant. And if its not gameplay but visual clutter that you complain about then I say that each challenge dungeon usually end in less than 50 seconds in a large group and all you can see is a mass of colors from all the spell effects anyway, an extra fireball or 20 won't hurt your eyes anymore than the casting spam that's already going on.