There is a huge issue in getting into raids for most players. Raids have declining participation rates that will only get lower as more and more people make these things exclusive. People are very rude and disrespectful to others in raids.
-Easier to fill
-More people doing them
-As a single person,you contribute 20% to the groups performance instead of 10%
-Less people you have to "herd" as a leader
-Slightly less interesting mechanics
There's really not many cons. The 10 man thing doesn't make raids feel more "epic" or "huge" or anything, it's just an arbitrary number that causes a much bigger barrier of entry for absolutely no reason.
The games combat does't even support raids, as boons started out at 5 people max. You've already started rebalancing entire specs and classes around giving boons to 10 people due to raids, instead of just making raids 5man.
Please re-examine raids player requirement and think of if the pro's outweigh the cons for 5 man vs10 man, anet.