Not surprised about this either.
When ISP-MIA and ISP-MSY were announced, I knew that they would both be short lived.
What surprises me is that they decided on a daily A321 ISP-RSW. I think 4x weekly on an A319 would have been enough.
As for ISP-MSP/DTW/ATL, I'm excited to see how this will end up. I think ATL and ORD will be a hit. MSP and DTW I'm a bit worried about. MSP and DTW are both primarily business destinations and not connecting opportunities, and on LI there aren't too many corporate businesses. DTW will be 3x weekly from ISP and MSP 4x weekly, ATL and ORD both being 1x daily.
As far as ISP-MIA, F9 does have very close competition with Southwest on ISP-FLL and to a lesser extent ISP-PBI. I haven't followed but possibly Southwest was aggressive in fare matching or discounting.
ATL/ORD/DTW/MSP have no comparable competition out of ISP.
DTW and MSP do draw leisure passengers who are going to/from their markets. Michigan is the 9th most populated state, while Georgia is 8th. A lot more credit tends to go to Atlanta over Detroit, but in reality, a lot of pax fly into DTW and drive elsewhere in Michigan, and similar in ATL for the Georgia and the Southeast. Atlanta isn't anymore a leisure destination, unless the Coca Cola factory is so great, nor is it warm in the Winter even though it's the South.
That said, I don't think the issue is the market choice, but on execution. If the service is offered at ideal time, like the daytime and likely atleast 5x weekly, it can draw more passengers.
MSP is a longer route over ATL and DTW. The problem is F9 is chintzy when it comes to dedication of resources to longer flights. For example, it does well in PHL, but the longest domestic route it will commit to is PHL-DEN.
The headscratcher for me is PHL-BHM and year round. I really don't know what the appeal is on either ends, unless it's a way for PHL pax to reach Atlanta region for a bargain basement fare and drive to Atlanta, or BHM pax to reach NYC via PHL. NYC is a draw in the summer, moreso than Philly for out of towners.