The only known cause of the molten metal is thermite.
No. Metal becomes liquid due to heat and heat alone. (not excluding pressure for pedants)
The problems that you have are numerous;
First is demonstrating that the material you see is Iron (Fe).
Second is demonstrating that this Iron is from the thermite reaction.
Third (if not Fe) is demonstrating that this material is molten structural steel (A34)
Fourth is demonstrating that this molten structural steel is the result of thermite.
Fifth is demonstrating that there is no other way that this material isn't either of the above.
There are an enormous number of metals/material that are present in the WTC and the aircraft that hit it that will melt and therefore flow
below the temperature at which Fe/steel/A34 will melt. If this is molten steel then one has to ask why is it not molten aluminium, lead or any other material.
You are coming to the conclusion before any proper analysis.
Lastly, lets assume just for the sake of it, that thermite was used to cut stantions and beams in the WTC -
What parts of the structure needed to be cut and how much thermite would be needed to make such cuts?
How does molten steel behave? If the fires were not hot enough to melt steel then how can molten steel not solidify quickly? There simply would not be enough heat nor shortness of time for molten steel to flow to an opening in the picture above. It would partially solidify and therefore not flow.
I don't know how much structural steel would need to be melted in various locations (to cause collapse) because the truth movement has never provided the calculations/answer.
If as you are implying by saying the material you observe is molten steel produced from the thermite reaction then you ought to come up with some figures with regard to how much steel was needed to be melted and therefore how much thermite was needed for the job.