I havent founf a reason why? dialogue doesnt say that you are the last. if they mean that the septim family is no more then couldnt the dragonborn have children who could have the dragonborn blood? lets assume my character is an imperial

even if dragonborn had children they woudn't be dragonborns, because being dragonborn means that the soul you have is the same as soul of the dragon, no new dragon soul in human means that there is no new dragonborn, and most likely now that alduin was defeted there is no need for new dragon soul to be placed in mortal .

Being Dragonborn is about having the blood of a dragon and so in theory their offspring would be dragonborn but since PCs tend to fade away and not settle down to raise a family this is unlikely. Also the prophecy at the end of the Book of the Dragonborn states this is the last.

Because there is no reason for another Dragonborn, Alduin was rejoined with Akatosh and the dragon fires no longer need to be lit to keep up the barrier between Nirn and Oblivion.

I never understood completely why the dragon fires don't need to be lit... Because Akatosh bit Dagon?

Because Martin became the stone that keeps the barrier up.

I always guessed that the gigant dragon statue replaced dragon fires as a local anti-daerda mcguffin

I'm curious as to what happens if the stone breaks. Will the barriers be shattered, the forces of Oblivion free the plague Tamriel with a terror none have seen in even the Oblivion Crisis?

That actually sounds like an awesome Elder Scrolls game/RP setting. With the barriers gone, tge Daedric Princes war amongst themselves to see who gets what on Nirn, and they all want everything. The mortals, I have almost no idea what happened to them.

Nirn fading into Oblivion wouldn't be thrilling for all of the Princes. Without it, they'll become dreadfully bored and what not. Bal and Dagon might not mind...at first anyway.