First of all these are entirely in speculation and what I have picked up on and may be entirely wrong and me over analyzing it.
1. They didn't immediately announce progress carry over would not happen
In the Halo Reach beta(ANOTHER BUNGIE BETA) they announced before it even started that progress would not carry over and they have made no statement that it wouldn't
2. Bungie can sometimes do crazy things
The game of destiny has hints to its existence all the way back in 2009 in ODST which had a sign on a wall saying "Destiny awaits" and a picture of earth with a floating orb near it. It was 5 years and no one even knew it was happening until recently.
3. They keep secrets when they are going to surprise us
Again no one even knew about destiny except them until some time in 2013. Another case of keeping secrets can be found all the way at the points where they really took off. In the cortana letters from 1999 there are hints to the halo trilogy story line up to halo 3. They already knew almost the entire general story 8 years in advance and somehow a few hundred employees managed to not leak a single thing until the time was right.
Whether or not they carry over progress they are still the greatest gaming company ever created and nearly ever single one of their titles has been amazing. If we lose our progress it doesn't matter that we lost a few days if on xbox and a week or two on playstation. I am sure this will be a great game that will be played nonstop for the next few years. So regardless of what happens we will not be disappointed as this is the start of something great and being a part of something great in the years to come
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Progress i would rather didn't carry over. My stuff in my storage and my money though. That would be cool.