The series should have ended with "Terra Prime". The ideal ending for Enterprise should have taken place like this:
Captain Archer returns to Enterprise after stopping Robocop from attacking Starfleet headquarters and destroying his base on Mars. Everyone welcomes him back to the bridge, expressions of relief and gratitude are exchanged. Captain Archer gives a heartfelt speech about how humanity is facing a moment in history that will determine its fate in universe, basically tl;dr slush. Then, as he is finishing his speech, there is a brilliant flash of light. Locutus:"I am Locutus of Borg. Resistance is futile. Your life as it has been is over. From this time forward, you will service...us."
Sam Beckett:"Oh boy.."