I mean they have a standing army more than a million strong. Each member of that army can wade into battle outnumbered 100:1 and still win. Not only that but they are IMMORTAL. Killing a guardian only pisses them off.
No worries your city is protected by millions of immortal juggernauts. With magical powers to boot.
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#Destiny
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What's stopping the fallen just dropping the equivalent of a nuke on the last city? No need to even set foot on earth and fight guardians. You can't really argue that the traveller would protect the earth, as he certainly isn't doing a very good job if all the Fallen running around Russia is anything to go by Ultimately, the vogons in hitch hikers guide to the galaxy had the best idea - they simply blew up the earth! Job done! And the shortest videogame ever :D