Russian proposals don't matter if the rebels don't agree to it whether U.S rejects it or not, I am sure you know this.
Russia is the only thing that doomed it to failure, not Iran and made it a non-solution when they got involved militarily.
Just because it isn't a solution because you don't like the results, do not make it a solution. I can tell you, your idea of a solution is also not a solution because not everyone will agree with the transition and an insurgency will still be present. There is no true solution in that deaths and fighting will stop even with an agreement you know.
I'm sorry, but you have no idea what you're talking about.
There was a proposal, with the support of Assad that explored a transition. This would have needed US support. You're not going to get stability unless the major powers agree on at the very least a basic understanding and actively with towards it.
It's not that I don't like the "solution", I'm telling you that it isn't one. You have Iranian interests in opposition to Saudi and Qatar. You have Russian interests in opposition to the US. Turkey and the Kurds. A mixture of militia and the Syrian government. Nusra, Al-Qaeda and others. Then to mess with all of that you have ISIS thrown into the mix with their own agenda.
Removing Assad is a small part of a larger problem. Syria is in essence the battleground for a multitude of wars or proxy wars. Assad has the full support of Russia and Iran with Iran Quads forces taking a very active role in logistical support. You have to involve these parties to explore the very miniscule opportunity of stability. There was one in 2012. Removing and bombing the regime isn't a solution because of the wider impact it has. The insurgency is going to be there for a while, that we can agree on. But right now the larger issue is dealing with the government itself and ISIS. You can't pin the blame on Russia here when there are so many actors playing the game.