Actually Fire isn't that much RNG. RNG is maybe if u do WR 100 or 10. But it's not that big that it puts u from the top to the bottom of the meters. With the upcoming change, they'll take away some RNG for sure but skill cap as well and imo way more skill cap than RNG.
So much firemages simply do not know how to build up combustions right. And if they are randomly lucky they're calling it RNG. But for all the argueing only the maybe 10% or less of the players that are doing it constantly right are taken into account. For all the others the RNG with combustion won't change, just the numbers were lower. But as combustions will be less part of the overalldamge this won't count as much as before and therefore it's less rng for them.