The game isn't locked but you lose out on a lot of content if you choose to not get the DLC. If you like Destiny that much, you'll buy the DLC. If you don't like it that much then just do the stuff that's still available.
If you can't afford it, trade some old games into Gamestop and get it.
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I know the whole game isn't locked. I was simply providing a scenario. Like the old saying goes: Give an inch, and they'll take a mile.