To expand on that, it's hinted all through their design schema. If they are doing this by accident or coincidence, that's amazing and good for us as long as they keep listening.
The game offers options for builds, but at the same time, most Archetypes have one or two 'signature' style abilities, that no one would really expect someone who wants to play as that class, to not take. Tanks will take Ultimate Defense 90% of the time. Clerics will take Radiant Burst unless they have an AoE augment from somewhere. Mages will take Black Hole or more likely Meteor Swarm. Other Archetypes are also each likely to have something like that. And if you somehow chose not to take that? Y'know, that's alright too, you might have a specialization that helps in some other content and just be expected to tell people you don't take it.
Limiting us to Primary Archetype abilities allows them to design encounters where 'one of each' is optimal. This was somewhat directly said by Steven (at the 19:00 mark) in
but I suppose it may seem like a stretch to interpret it that way, for some.
These 'signature skills' allow them a lot of clarity in design. You can make it so that if there's no Tank with Ultimate Defense, some appropriate mechanic will make the group's life
really hard
. (
@Vhaeyne
this part is more for
@Ugoogee
than for you, I'm sure you're familiar already)
Hybrid Combat is also not guaranteed to be 'everyone will accept the style of play you use', just that you will be able to play it and succeed. And hurtboxes on enemies, that move around, allow you to make it so that Tab players do what they expect to do; 'mostly stand in one place and wait for an opportunity', and Action players do what they expect to do; 'get into position and aim their attacks'. This is another reason why we were told up front that players will 'have to' take 75-25 split. You have to become familiar with things, because balance is going to involve you 'moving toward doing at least a little Action Combat', and optimization is going to involve you 'taking advantage of the speed of Tab Target' (usually for defensive stuff I'll assume).
Designers also don't have to work that hard, given the Archetypes that we have. Clerics can do damage, but not a lot of 'high sudden damage to specific hurtboxes', so you can't necessarily just bring a ton of Clerics. Tanks are also slightly limited in this, so 'All Tanks and Clerics' would might manage but it wouldn't be easy. 'Almost all Fighters' has a different problem. Even if they are built tanky, Bleed and Poison and a few other things don't care about this, only Ultimate Defense meaningfully does. And Fighters probably don't naturally get much if any strong debuffs that are likely to work well on raid or even single-group bosses. It's more likely that their 'signature move' is the gap closer, and their role is actually damage. It's unlikely you'll need to worry about Fighters finding spots, but if you fill everything with them for 'more damage', you don't have 'enough' control. They have to stay 'below the hate line' established by the Tank, but they're not helping the Tank 'avoid losing hate to keep that hate line high' in most cases.
Which brings us back to the 'Rathalos'.
Poison. Burn AoE on the ground, relatively powerful bites (it's a very 'output heavy' enemy generally). You want this blinded. Maybe you want it Paralyzed so that it sometimes fails a full strength Fireball breath. You need someone (probably just the Tank though) to put the 'damage mitigation' on it. You probably want a Bard to keep the Tanks in a specific buff state and Summoners and Rangers to help keep the enemy in a specific debuff state (don't overwork your Bards!). Rogues and Fighters bring the damage, handle Adds of either the PvP or PvE variety, and target vulnerable hurtboxes to change the fight or disable enemy mechanics.
And you need the Mages (or certain Summoners) to stop it when it
flies
. If you don't have a Prismatic Beam to blast at its head, or a Black Hole to pull it down, then you have to 'endure' the flight stage (or hope your Tank or Cleric is good at hitting a moving head with some Flash ability) and the 'crash down' threat or whatever else. You could treat it like the Poison Dragon in Siege, and just have everyone 'be perfect and avoid the touchdown damage', but if it's now out of reach of melee and still fireballing and similar, the Tank now has to play much more Action. They're not doing damage (probably still have a Threat ability but maybe fewer that reach) but lose threat as they take damage. Augments help Tanks when their abilities can actually hit. Ever wondered why Tank doesn't just have a 'use this, get Threat unconditionally' ability yet? I don't.
If someone isn't on point, danger increases rapidly, and that gets 'worse' if the party comp isn't right. Or maybe not quite 'worse', because you could just, again, balance it. Fighters might just 'not care as much' about what is happening when it is flying, but they still lose some effectiveness. The fight takes longer, the group uses more mana to reach the same point as a more balanced group, etc. You can make do with any 'Trinity' combination, but you have a reason to at least seek out one of each, and even if that one doesn't have every ability you expect, they probably have something.
This isn't just 'a Rathalos thing'. This is a formula, it's why Monster Hunter works. I can do this for basically any enemy because Ashes is built for it.