Just like 5v5 ranked team queue before it, playing with people you know doesn't necessarily guarantee you a win. That is, if you're making the assumption that all individuals are equal at your rank.
The teamwork factor can make a huge difference, but if you're playing against individuals who are objectively better than each one of your teammates, more often than not, this will result in a loss for you.
Edit: Concerning the content of the edited question: yes, there may be some inflation of rank, however, you must keep in mind that Riot restricts the rank gap of players queuing ranked together.
The only restriction on your Ranked group is based on rank. You must be within two tiers of the rest of your team to play in a dynamic group. This means, for example, if you're in Gold, you can only play with those who are in Silver, Gold, or Platinum. Unranked players may play with Bronze, Silver, and Gold players.
Also, I think that your assumption that rank was a solid indicator of personal skill prior to dynamic queue is a bit misguided. Prior to the ranked queueing rank gap restriction, people were being boost simply via duo-queueing. Since the gap restriction, I feel like things may be slightly more accurate, but there are alway loopholes. Very highly ranked players can simply make smurf accounts now that are unranked and boost a Bronze player all the way to Platinum.
I think that Riot's intent with this queue is to really force people to start thinking and behaving like a team. It isn't a stretch to say that, historically, in solo queue, there has been a mindset of "I have to carry, I have to do the most damage, I have to have the best score, if I don't carry, we will lose".
This thinking is detrimental to the team dynamic and (IMO) is the source of all the negativity people have for their own teammates. The old queue promotes that 1v9 mentality where a player may feel that it's his effort vs the efforts of everyone else in the game.