One problem with comparing DP to GSC and RBY is the amount of options available at the time each game was created.
I mean, it's fair to criticize the fourth generation games for such a flaw when it wasn't there to the same degree in the third generation, even though it was also present in the first and second. It's one of those "they really ought to have fixed it by now" type of things. By the time Gen IV rolled around, there were almost twice as many Pokémon available as in Gen II, and that also goes for the number of families of the uncommon types. The first two generations were held back by a lack of options, which really wasn't the case in Gen IV. So DP having poor distribution of a Pokémon type is a worse case of planning than when it happened in GS.
Or put another way: We give Gen I's lack of Ghost types a pass because there was only one Ghost-type family back then. Gen IV had 13 non-Legendary Fire-type families available (excluding Castform-Fire), which makes it very baffling that only two were available, one of which was a starter.
While comparing DP to GSC and RBY isn't perfect for the reason of limited options but was intended to prove a trend, the criticisms as far as Gen III are concerned are still valid. Gen III gave you the option of 3 Ice-Type families (excluding Castform-Hail), which considering one is the hard-to-obtain legendary Pokémon Regice is functionally the same as Fire-Type deals with in DP. It also still only gives players the option of 3 Dragon-Type Pokémon, with one unobtainable until after the 8th gym badge which is too late so is functionally 2 Dragon-Type Pokémon, and 3/4 depending on version Ghost-Type families but one is Shedinja which is so frustrating to use in-game it barely counts. It also provides 3 Steel-Type families total, which I don't think is much better than 2.
The overarching query I have with the argument is why Gen IV specifically is the tipping point. Why not Gen III, which faces the same type distribution issue with more types than DP does? Is it truly just that Fire-Type is a starter type. so players care more about it than Ice-, Dragon-, Ghost and Steel-type Pokémon? Because if a Gen III player happens to have one of those four as their favourite type, that's not much consolation. And it affects the player experience regardless, since besides Sidney those 4 types being so rare hurts the Elite Four and Champion experience.
RSE had more than enough Pokémon families to represent all of the types adequately, but there was a choice not to. GSC decided to lock Slugma, Murkrow, Misdreavus and Houndour in the post-game even though Dark-, Ghost- and Fire-Type Pokémon were underrepresented in the main game. They also chose to for some reason make Mankey version exclusive to limit Fighting-Type options even more in one version than the other, and to make Rock-Type Pokémon difficult to obtain besides Geodude, Sudowoodo and the unusable Shuckle.
The argument of context is flimsy to begin with, because designers at Game Freak have spoken about game directors giving them orders to create a Pokémon of "X/Y type". We didn't get the first Fire/Bug, Psychic/Bug and Ice/Bug Pokémon at the same time in SwSh by accident: the creature designers were told to make new type combinations for Bug-Type and the drafts that were liked were finalised into Pokémon (I read this in an interview but I'm about to sleep so I'm not digging it up now). I'm sure that the designers were also told to make several Dark-Type families in Gen III, making the type more common. The same thing could have been done for other types lacking in numbers.
Although even in the context of the Pokémon available to include in the games in Gens II and III, they made bad choices in terms of type distribution. Pokémon XD: Gales of Darkness' encounter table's 83 Pokémon families do type distribution better than or equal to RSE's 92 (discounting hard to obtain legendaries, 102 counting them). XD has 7 Ice-Type families, 4 Steel-type families, 3 Ghost-Type families, and 3 Dragon-Type families compared with RSE's 2, 3, 3/4 (but one is Shedinja), and 3 (but one is post 8th badge) respectively . And that's working within the constraints of not having trade evolutions available, removing Kingdra from the pool of potential Dragon-Type Pokémon and Gengar from the Ghost-Type one, including no gimmick Pokémon whatsoever, and not wanting to double down on key encounters in Colosseum removing Misdreavus from the Ghost-Type pool and Skarmory and Metagross from the Steel-Type one.
And this wasn't because they had the option to make all types as rare as each other and remove the "rare" quality of certain types. XD still has 9 Bug-Type, 11 Flying-Type, and 13 Normal-Type families for instance. They largely mirrored the traditional commonality of types, with the main exceptions being the over-representation of Fire-Type and under-representation of Water-Type families with 8 of each.
If XD: Gales of Darkness could do it right in the same generation as RSE, then the number of Pokémon available to Game Freak isn't an excuse. They were just bad at balancing type distribution until Gen V, where they upped their game to adequate in BW before circumventing the issue entirely by throwing in as many Pokémon as possible into future games.
It's not a Gen IV exclusive issue, and it's clear that that's the case. Game Freak has been bad at it all along.