Why i think Shepard's story isn't finished.

For the guy to tell the story, and know the details about what happened on the Citadel, someone had to have been there, and survive, to tell the first story.

The old man and the boy scene was referring to stories the child hadn't heard yet. Not the player. Imo. I don't believe it means Shepard story is not over for us.

Even though the reapers are destroyed, how to they know the threat has been eliminated?

I think if you choose destroy then a somewhat beat up Shepard goes into retirement, sitting on a beach somewhere sipping cocktails and reminiscing on old times with Garrus (also retired) and whoever else wants to join them. After all I think they earned it.

However, if the Council chose to involve them again it would likely be in some political capacity on a diplomatic mission to sort out problems that arise in the new galactic order. Having proved themselves adept at uniting disparate groups and overcoming age old animosities, naturally Shepard seems the ideal fit for the task. Then what seemed a simple mission turns out to be something more.

From my personal perspective I just assumed that the Citadel DLC actually took place post-destroy instead of in the middle of the war. It made sense that the surviving Cerberus people would want to replace Shepard with a clone so they could capitalise on their reputation and it seemed far more appropriate to have a big party to celebrate removing the ancient threat to the galaxy rather than whilst we were still fighting for our lives. So everyone was invited round, it was clear that EDI hadn't died as a result of my actions, we had a good time, the end.

I have played through full ME series once and chose the Destroy ending. First there was a scene where they put Shepard's name on the plaque, so, the game characters presume she's dead. HOWEVER, there was a scene following that one. It depicted a giant pile or rubble on some planet and in that pile of rubble I saw that rectangular locket thingy that belonged to Shepard. Then the last shot of the game was how the pile suddenly shifted. I had a feeling that Shepard was going to push the blocks aside and crawl out, but of course then the screen went dark. So, I think they are giving you a chance to imagine she survives.

The most annoying part of the plotlines for me was that I found the array that allowed us to build in a specific Reaper only targeting ability into the weapon (I think this may have only been once I had the Leviathan DLC) and then I was forced into either allowing the Geth to have uploaded Reaper tech from Legion OR destroy them. Allowing the upload really made no sense at all, particularly if you had just done the side-quest where you are specifically removing a Reaper virus from the Geth. So naturally if allowed my true choice, it would have been to deny Legion the upload but still broker the peace between the Geth and the Qarians. Of course that would have made them proof against any Reaper only targeted gun.

Sadly that was not an option, so first run I went with the upload against my better judgement, half expecting the Geth to turn on me at a later stage. When this didn't happen I felt elated. I had actually pulled off uniting ALL the galaxy, AIs as well as Organics, into a collective fighting force against the Reapers. Then I got to the end and I am told that using my weapon against the Reapers will destroy all AIs, which I felt was a pretty cheap trick for the writers to play. I also hated that they made destroy the red option, implying that only a totally ruthless Shepard would take it. However, having seen the result of taking the "preferred" (by the writers) green option as a "reward" for my efforts, where essentially everyone is turned into pseudo Reapers, I find destroy is the only ending that leaves me with any sense of accomplishment, regardless of whether Shepard survives or not.

EDI told me she would be happy to die to end the Reaper threat and I felt the same way. Before the Extended Cut it seemed likely that taking Destroy would also result in the end of Shepard so I really wasn't expecting their story to continue.

From the beginning of ME1 up to before Shepard goes up the beam in London, there have been eyewitnesses and even Shepard, saying what happened. But who is the one to witness what happened on the Citadel, if Shepard is dead? Shepard survives, if destroy is chosen and ems is high enough. So, it's obvious he/she would tell the first story. 

Another thing is the guy says the details have changed over time. Those details could be anything. It could be that Shepard did survive, but people got tired of hearing that story, so they added some spice to it by including green and blue. I'm sure the story told to the guy is a lot different from the story he told the kid. Another thing is, was there another person with the guy when the story was told? How different would that person's story be from what the guy told the kid?

I had a weird idea that because Shepard's "essence" was dispersed in the Synthesis explosion/reaction, some portion of his consciousness was sort of running life in the galaxy. As such he could sort of appear to people in times of crisis and act as a kind of spirt guide - like the Jedi spirits almost.

I always liked the Man of Steel depiction of an uploaded consciousness to an AI as well for a Control ending...

This is my general belief as well.  Shepard's story was told.  It starts: Earthborn/Spacer/Colonist.  Next: War Hero/Sole Survivor/Butcher of Torfan.  First human Spectre.  Takes down Saren.  Allies with the enemy to stop the Collectors.  Returns to the Alliance to face the Reapers and wins (or not, if you choose Refusal).  Shepard got a hero's story.  Anything more would just muck it up, diluting Shepard.

I like to see Shepard return. It doesn't mean that I agree that without him/her it wouldn't be an ME game.