From
this article in the Orlando Business Journal.
The 135,918 passengers who took Brightline to Orlando is equivalent to 22 flights per day (assuming aircraft with 200 seats).
If memory serves me correctly from when I last looked into it, this is about the same number of flights to Orlando from the three main South Florida airports. I'm sure there has been a reduction in the demand for those flights, but it was probably pretty minor. Most of these train trips were probably latent demand.
I think the success of the leg to Orlando bodes well for the demand if the service extension is ever built.