Especially considering that literally no final evolution of a Fire-type starter is a quadruped.
That's kind of the point, isn't it?
More than two thirds of all Fire-type starters have a humanoid or human-like design in their final evolution (slightly fewer if you don't count Infernape). Arguing that there should be a quadruped Fire starter isn't an argument based on precedence and traditions,
it's an argument that they shouldn't keep repeating the same old thing over and over and over. Many fans, myself included, are tired of every Fire starter having an entirely animal design base for the first one or two stages, then mixing that animal with a human profession in its final stage. Why the constant need to make the final evolution humanoid? They're taking it far, too. It's not even like the starter is a human-like animal, it's more of an animal-like human. Or a "human in a costume", as you could describe them.
Litten could have stayed feline, no need for it to become a (human) wrestler. Fennekin could have stayed a fox, no need for it to become a (human) magician. Tepig could have stayed a pig or a boar, no need to make it a (human) sumo wrestler. Arguably, Infernape is fine (but for the typing), it is decidedly more animal-like than human-like. And Blaziken, well, gets a pass for being the first to start the trend. Fire/Fighting was original in its first inception, it didn't get stale until it was repeated.
Worst of all in my opinion, it seems like the trend is spreading. It was fine enough in Gen V, where the other starters were a snake and an otter, with little to no "human touch". But in Gen VI, suddenly the other starters turned into a knight and a ninja, both of which are human professions. And in Gen VII they're an archer and a mermaid/singer. At this rate, I'm expecting all the Gen VIII starters to also become human-like upon evolution, no matter what their first stages look like.
I prefer that at least some starters stay beastly, rather than always resorting to a human design basis. The number of legs isn't important. Sceptile, Charizard and Empoleon are all bipedal, and they are perfectly fine. Their designs are diverse, with different postures, design bases and abilities. The world is full of bipedal creatures that aren't humans, and the designers have done a good job of utilizing them when it comes to starters in the past. But lately, it's as if they want them all to have an upright posture, opposable thumbs, cheeks and eyebrows. The only excuse I can come up with is that it'd be easier for kids in play to pretend that they're Pokémon. The posture of the Gen I starters didn't stop us in the late nineties, though.