Game wise? *shrug* It's like, whatever. But if I were to extend it to existence in general (which can only be viewed retrospectively), I'd be sad if the Nidoran family didn't exist. It's hard to really say, because again, this can only be said retrospectively. If they never existed, it wouldn't have mattered, and it's likely my original favorite, Vulpix, would just take their place.
But assuming all other things played out as they did? I wouldn't be into Pokemon anymore. Vulpix is an extremely high favorite, certainly in the top 3, but they don't fill me with much enthusiasm. The only Pokemon other than Nidos to do that was Kirlia and Mawile back when R/S was announced, and even then I burnt myself out on them eventually. Without Nidos I would have quit Pokemon altogether after Generation III, which (retrospectively), would have been a shame because I would have missed out on the gems that are the sixth generation, and by extension those that I missed from generation V. The greater majority of generation IV is just far too "meh" for me, they don't invoke any feeling, which is the worst they can possibly be. They say the opposite of love isn't hate, but apathy.
So, all that said, given my enthusiasm for the Nidoran family (bar Nidoking, sorry man), it really was on them to keep me at least tangentially interested in Pokemon as a franchise during my Pokemon burn-out between the tail end of Gen III and the beginning of Gen VI, odd as that sounds. So there you go.
TBH though to say I couldn't live without them is really pushing the hyperbole. I'd honestly be fine without them, either way, but here we are.