I think the reason why it didn't feel like a series finale is because the villain of the series finale was not UBER A. You would expect from the final villain to be the biggest, baddest villain of them all who has been in charge of everything since the beginning. Alex wasn't. She was just there to give us season 6 and 7. Otherwise CeCe would have been revealed in 5x25 and the show would have ended with Big A.
Yeah that’s a good point. They should’ve never called Alex “Uber A”. Heck, they should’ve never called her an A. She never even wore the hoodie in the finale, what the hell. Alex had her own agenda. She’s just another villain, but not necessarily an A.