You are correct. Long before it actually melts, a steel support beam will lose most of its strength.
I've done blacksmithing (on an amateur basis). At red-hot heat you can shape steel with a hammer (although it's easiest at yellow-heat). Working steel at yellow heat can be a bit tricky, because if you over-do it and get it to white heat, the steel will start to burn. I did that once -- the piece started shooting sparks like a 4th of July sparkler. And that was WELL below actual melting point.
At the temps inside the WTC, once the support beams got past red-hot, it was going to come down
I can see the possibility of steel getting so hot that weakness does occur causing some type of collapse but the video I've seen shows a simultaneous, symmetrical collapse........no, I'm not an engineer but how could all the supports fail at the same time.
Now try and convince you average conspiracy nut about the inherent logic in your last sentence there. They are completely blind to it.