Well, first, you need an alternative damage type in case fire doesn't work, so lighting was a good candidate for that (why
doesn't lighting have faith scaling on weapons then?). Second, lighting in this game is associated with fire. Gywn, the Lord of "Cinders", Kiln of the "First Flame", whose soul can grant the player the "Sunlight" Spear, which does lighting damage though the sun is bourne out of fire etc. There's an association between the first flame and the fire of pyromancy, which if i remember was a result of the witches attempting to create a second flame.
Second, i would suggest that
at the very least, there probably was an expectation that a pyromancer was going to be lower in level than other classes. That's because the player is given the choice to use souls either for their character or for the pyro glove, and it's not a given that everybody has the surplus necessary to level up both at the same time, certainly not in the early game. Even if they didn't think about the possibility of not leveling the pyro at all, there was at least an expectation a trade off had to be made, hence the need to bolster the resulting weakness in character level by offering a weapon infusion that didn't hinge on stat scaling. From there, it's a very small step to make from deciding not to level at all, or see how far one could get if one prioritizes the hand exclusively (as it turns out, you can make it all the way to the end).
The fact remains that Dark Souls 1 is the only Souls game where you can end up with a SL1 character that
isn't in fact that far behind a regular character in terms of power.
As for PvP, i'm not entirely sure how much that would factor in here. I certainly don't think they expected twinks and it's not like the pyro is the only class that can be twinked out to harass low level players. I would also assume PvE was their first priority when designing the game, hence why it never even occurred to them to have a matchmaking for the weapon upgrades originally. They weren't expecting people would create low level characters with maxed weapons to go harass low level players.
There is also the attunement mechanic, which meant that if you wanted more than 2-3 spells you needed to level it up. The fact that combustion spells were broken as shit doesn't mean From designed the system with the idea that the player would only use 2 spell at the time. Add the fact that Dex increases casting spell and there is even more reason to think that they did wanted the player to level up even if they are using pyromancy.
It's not that they didn't "want" the player to level the pyro. It's more that they allowed for that possibility, which is a logical possibility given the inherent mechanism of the flame glove and the lore behind it. I mean i made that assumption way back then and back in the day i had zero clues SL1 runs were a thing and basically had no idea how anything in the game even worked. I just saw the pyro glove, saw that the pyro started at SL1 and instantly made the assumption it was a kind of alternative playstyle where you leveled the hand instead of the character. It just feels like an obvious leap of logic to make, especially since this was game known to be "very hard" so why wouldn't they tempt or dare the player to make an hard core choice (while making sure that option was still viable).
I mean, ultimately, ask yourself this. Why wasn't the deprived the SL1 class? Why the pyro specifically? If you don't consider the existence of the pyro hand and the way it works, one could say the choice was kinda of arbitrary. For all intended purposes, you could even say they had an expectation that people were going to try to do SL1 challenges, and normally they would pick the deprived to do it (as they do in all the other Souls games). So instead of making the deprived the SL1 class, they picked the pyro, the one class that can still be viable even at SL1, to sort of "soften" the path of trying to beat the game with a low level character.
