Given the rampant vandalism of Churches in France in recent years, including arson, it's quite reasonable to speculate that Notre Dame might have suffered the same fate.
Given the lack of evidence for arson, that speculation seems quite unfounded. Asking the question "Who set the church on fire?" is also not merely speculation, it is a presumption.
Purely a matter of personal opinion. There is a clear trend at play in France that is very uncomfortable and disturbing. I think it's lunacy to ignore it. But to each their own.
That such speculation is often met with animosity is strange in itself.
The conspiracy theory, if there is one, is that there is evidence of arson, but it's being suppressed by the authorities. I'm explicitly not taking a stance on that.
Again, I have no problem with speculation. The question, however, was not speculation. It was starting from the assumption that the fire was intentionally set. That is not speculation.