Commuter trains and trains with a more intercity* focus like Amtrak and Brightline do serve mostly different markets, but there is an overlap. There can also be an assistance to each.
Yes, the prices are different, as are the services, but some people will use either service. In the southern Florida area, Tri-Rail will serve many more locations, giving them some advantage. This could mean that passengers ride Tri-Rail to a station near a Brightline station and then swap trains. I do this now in the New York, Chicago, Los Angeles, and Miami areas routinely.
I can see Brightline competing with Amtrak on the Miami-Orlando route, and with Tri-Rail south of West Palm Beach. However, with the routes apart from each other, and the focus being different, it will be that 10% overlap that will be interesting to watch.
*And on the issue of Intercity routes, many commuter routes are now of distances that they can probably also be called intercity. Some are as long as 2 hours and cover multiple states. Commuter agencies now routinely have cars with large baggage racks, restrooms, and wifi service. This can create a challenge for Amtrak and other more traditional intercity rail providers.