I think we both know that's not true. Objectively, they have increased their vote share and are still the biggest party by quite a way, even if the lost seats. Their support isn't exactly crumbling before our eyes.
May IS done, though. Her shelf life is short. Her sole task now is to expend any political capital she has left to negotiate Brexit, or get as far as she can, then step aside for someone else. If there's one thing that The Tories are good at it is regenerating themselves, replacing ineffective leaders with more effective ones in order to fight future elections.