There's more issues. For their ATL-based operation (and the BIL-based, which is going away), they were having to rent another airline's reservation system since they didn't have one of their own. Once they got their own, it opened up the possibility to operate the Florida flights on their own and eventually probably code-share with DL on them as well as UA.

Additionally, I don't know how much UA liked having UAX outstations in places like BIM, ELH, etc. I think they would much rather have Silver deal with the whole operation there and not have any responsibility for the pax until they get to Florida.

Staffing would be another issue. For example, UA runs a pretty thin operation at PBI in the summer. Silver's flight schedule there was to some extent limited to when UA could handle them.

In IAD and CLE it makes sense to stay UAX because the routes are nearly 100% feed from UA and they can cross-utilize UA's much larger check-in and pax handling facilities.