Nothing is actually stopping small guilds from unlocking these missions, it will just take more time. You make is sound as if you CAN'T unlock them at all unless your guild count is over a specific number.
So much this. Nobody is equal. Just like players who can play 8 hours a day will have much more money than players who play 1-2 hours a day. Some people got their precursors on first try, some people threw hunders of gold into the mystic toilet and didn't get anything back.
Personally I wouldn't really call a guild with less than 10 people a guild (no offense). They are more like a group, party or rather: a family. They can easily all gather up in any zone and do events together or just do dungeons without leaving out anyone... while large guilds can't really get everyone together as dungeons are 5-mans, and in zones they're much like a big zerg (which is not that fun).
Already 6 months passed since release and big guild had nothing really to do together (unless if it's a WvW one, but I'm a PvE person so I don't care). Sure they could gather up and do some crazy, pointless stuff but sometimes it could've been better if they can do fun things that's also rewarding. So I don't know why the small guild are so angry, it's basicly the big guilds that didn't get much love until now. And it's not like smaller guilds can't do guild missions, sure it'll take them a while to unlock everything but it's not that big deal imo.