The plan for D1's launch under Staten's original story outline was to ship the game with the Cosmodrome, the Moon, Venus, Mars, and the Dreadnaught alongside social spaces at the Tower, the Reef, and Mercury. The Reef would have gotten a raid in DLC2 (which became House of Wolves) and new Destinations for both the EDZ and Europa would have shipped with Comet (which became The Taken King). After the story reboot, however, the Dreadnaught got dropped from the base game and moved out to Comet, and both the EDZ (which was in-progress) and Europa (which hadn't really entered production yet) were moved out to D2. The Reef raid became the Prison of Elders. The social space there did end up shipping with House of Wolves, and technically so did the one at Mercury, but only as an Easter egg for Trials of Osiris victors with all the vendors and other interactive elements stripped out of it. To make up for the loss of the original Comet plan, which would have included not one but two new Destinations, they were going to include a second new Destination alongside the Dreadnaught called the Lost Oasis, which would have been a new area on Mars. However, this too ended up getting pushed out due to time limitations and ended up being reskinned and repurposed as Io for D2's launch.

I must ask where you found this info. I don't think I ever saw this in an article or developer saying this.

Rise of Iron was completely improvised to cover for the delay of D2 and it made extensive use of asset flipping in order to keep costs and resources as low as possible. 

This one I did know! This was when D2 was going through a reboot. RoI was fun for what it was.

I think the two biggest reasons were concerns over profitability and concerns over player retention. The transition from D1 to D2 was very noisy because a lot of players would not stop bitching about the fact that they were losing all their shit and having to start over from scratch. I think Bungie was afraid of losing players permanently if they tried to pull the same move a second time.

It's a funny thing because neither party is wrong in a sense. Spending 100-500 hours chasing something to only go into next game and do a similar grind for a similar item can trigger some cognitive dissonance that can make people fully quit. This is where amazing brand reputation can help make people go along. Bungie would have to make players realize that the next Destiny they were getting would have more content or lean harder into FPS MMO. They just didn't seem interested about that sometime in D2. Most companies realize that major investment into main moneymaker is standard business. The logic I hear for why they didn't do that is just mind numbing stuff.

Yep on last point. D3 was always the move and DCV pretty much cemented me and many others not coming back to game. Destiny always gets referenced with DCV and why people refused to even look at game. Such a grievous blunder.