Approximately 30% of the players that play league are in Gold or higher. This was a bit lower at the start of the season (approximately 25%) and I think last season at this point the figure was also lower.
But, just that fact that 70% of the player base is in silver or bronze means that most players that achieve gold are either OK/Good at the game, play a lot, or/and are boosted.
I have moved a bit higher in gold lately and I have noticed several Gold Vs playing really bad. But they aren't playing atrociously bad, they just make a few mistakes that lead to huge deficits while laning. Better players are able to capitalize on those mistakes, whereas in Silver or Bronze they will not and games can remain even for a really long time. I also think, as someone stated earlier, that it could be that the strategies and tactics a player used to achieve Gold no longer works effectively in that Elo. I can get away with a lot more playing against Silver players in a normal than I can in a ranked game among my peers in terms of skill.
It could be that these Gold V players just need to adjust their strategies and they have yet to do so. I have had several friends achieve Gold and lose it. They didn't make the necessary adjustments to improve and float between high silver and low Gold. That may be where their skill is and the only reason they achieved gold was due to a hot streak.
I don't think the prevalence of boosted players can be that high if only 30% of the player base on NA can achieve Gold V or higher. I would expect this figure to be a lot higher otherwise (40 or 50%).
Unless your underlying assumption is that the people that play league well is actually very small and perhaps only half of those players are "true gold" players. I don't know what the ratios were of Gold or higher players were in previous seasons before 5, but i think at the end of the season it was at a pretty similar figure.
People also may be obtaining gold "easier" because of role selection. I may have achieved Gold III this season, but I probably can only ADC or Jungle at low Silver Level.
With new champion select, I frequently get my main role more often than last season. I received top probably around half of my games last season but the figure is much higher this season. It could be that several low gold players are "one-trick ponies" like myself. They play their main role well and play their secondary role only so-so. They could be getting their secondary role more often in games and simply just stink at it. Especially if their secondary role was support. As we all know, placing support as your secondary simply means you will get support. They may have decided to try to learn a new role because they were tired of getting support all the time. They may completely stink at that role.
I have had to support more often this season so I got better at it. But last season if I got support the bot lane would probably lose because I was terrible at it (I was better at mid).
This doesn't even account for those with second accounts that achieve gold and maybe their main is platinum or higher. My smurf account is Silver I and may reach gold by the end of the season. I never intended it to get this high and only created it so that I could play ranked with some of my friends without taking them into games with Gold players. They would simply get stomped.
I also think it a bit easier to move up with Riot giving you wins if you keep repeating the same promos, but I don't think that is a huge factor in being boosted.
There is not a dramatic difference between Silver II/I players and those that are Gold V/IV. The difference may be grinding, a bit of luck, and getting your main role several games in a row. The reality is that Riot made it a bit easier to achieve gold if someone plays a lot. The new champion select, the ability to get your main more often and not play roles you stink at, and the ability to queue with friends.
But if the percentages of those that achieve gold is comparable to last season (which it is pretty similar) there are fewer people that are boosted than one thinks.