"Fire based damage is the least you want after Twin Princes. That's a fact".
No, it's not a fact. There are 5-8 bosses classifiable as endgame bosses after the Twin Princes. First you have the Nameless King, an optional boss that can be defeated even before the TP fight, but I'm just including it in this category nonetheless. He has two phases, King of the Storm phase and his own phase. The stormdrake in the first phase is resistant to fire, and therefore lightning is the ideal element. However, the more difficult phase is the next one, which is against the King himself. The NK is resistant to lightning and is weak against fire. Therefore, any fire based weapon will serve perfectly well against him (including the LG, provided of course you are comfortable with using a slower weapon like a UG).
Next up are the SoC, Demon Prince and Midir. All of whom are resistant to fire. Your point holds good here.
Then you have Halflight, who is a minor NPC style boss. Literally any type of weapon can kill him, especially a hard hitting one like Lorian's. He is also weak to fire, so double cheers for fire weapons.
Finally you have Gael, who is not resistant to Fire nor does he possess any specific weakness. However, since his resistance is not significant, any fire weapon does just fine. In fact, any weapon that is not Dark or Magic based does perfectly well against him.
You also have the AoA bosses, Friede and the Gravetender (which you can do any time before or after the Princes, but given Friede's difficulty I am classifying her as a late game boss), Gravetender is weak to fire. Friede wields no significant resistance against fire, so fire works just fine. In her second phase, Father Ariandel is a being who is weak to fire. As such a fire weapon makes the overall fight easier by letting you kill him quickly and thus putting you at less of a risk of being destroyed by Friede in or before Phase 3, As such, fire weapons are great for the fight as a whole>
Bosses aside, you have the mobs. Most of the mobs in the game (early game or late game) are very susceptible to fire. Fire is easily one of the best weapons against a standard enemy. Some are resistant, but most are weak or have no notable resistance. As such, fire damage is very much useful.
So no, I would disagree with your point. Fire damage is still very well useful in lategame. In cases where it's not, then just switch to another weapon that is suitable for the scenario. In all my playthroughs I do just that. No single weapon in the game is going to be the perfect one for any and all situations. Expecting it to be so is ridiculous.