Are non-legendary Fire-types that worthless?

They are still as good as they used to be... but there are slightly better attackers that have slipped in ahead of them. If you have not powered up a legendary fire attacker (I haven't yet) then don't feel bad about using one of the top non-legendary fire pokemon.

They only received a minor power creep imo. The ones u mentioned are still useful attackers if u have invested in them. But a non-legendary fire type is coming in gen 5 that will oust these existing legendary fire pokemon

Fire-type Pokémon will find their rightful place in the meta once Gen 3's menacing Metagross and Aggron make their appearances. The only Fire-type Pokémon even worth mentioning right now are Moltres and Entei, but that's only natural, since they're legendaries.

Flareon with FS/OH is the 5th highest DPS currently.

I don't think that they are worthless, it's just that there are so many of them without one really distinguishing enough to be separated out. I have a 100% fire fang/fire blast Arcanine that I maxed out to level 36.5 that performs very well and rates high on pokebattler for raids but he will never be listed as a top counter or attacker because too many options exist of the same type that are just as good or better.

I found out that my WC Arcanine is a great Gyarados/Articuno/Scyther counter lol.

They aren't useless, it's just that the legendaries have the ability to be better. However being better is largely a moot point, since basically all fire pokemon are useless in the current meta (based on the dearth of grass pokemon used by players most of the time). I personally haven't bothered to power up any moltres of entei yet since there is no need, my flareon, arcanine and charizard (which are all already powered up) do just fine in the few situations I need to use them.