Seeing Herma-Mora pop up in Tamriel to go all tentacle on Storn got me thinking. Granted, this was probably not Herma-Mora's physical form (like Mehrunes Dagon in Oblivion) and maybe an aspect (like Hircine's in Bloodmoon).

But since there isn't a Septim heir anymore to wear the Amulet of Kings and the Dragonfires are not lit anymore -> why hasn't any Daedric Prince (and in particular, Molag Bal and Mehrunes Dagon) invaded Tamriel anymore ever since Oblivion ended? Is there still anything deterring them?

Or was Herma-Mora merely able to enter Tamriel because this was during a prophesized event likely mentioned in an Elder Scroll? IIRC Hircine was able to physically manifest during Bloodmoon, but chose to fight the Nerevarine as an aspect.

When Martin broke the Amulet and mixed his blood with it becoming the avatar of Akatosh - he sealed the barrier... permanently.

Of course, there is only so much you can seal, the Daedra will always find a way through, but there will never be a Daedric invasion reminiscent of the Oblivion crisis(or ESO's Bal's anchoring I imagine)

The Principalities can still make cameo appearances like Mora and Sanguine, but not at their apex power like Dagon could.

Martin's apotheosis acts as a placeholder for the Amulet/Dragonfires.

Storn died because he allowed himself to. He held a dangerous item and essentially gave Mora consent to physically assault him. Mora could not have otherwise entered Nirn.