I started writing to address you point by point. I got about 1000 words in and then I realized that you really have zero perspective on what is going on in Syria nor do you understand the implications of what would happen given you hypothetical.
So first, ISIS is losing on all fronts. I'm not going to bother bringing up maps, so you can go to /r/syriancivilwar if you want to search for territory control maps.
Secondly, current US strategy in Syria is working... to defeat ISIS. As well as in Iraq. This leaves other factions mostly to the Russians and the SAA.
We aren't talking about ISIS. I had no idea you were writing under the mindframe that this force was meant to defeat ISIS. Now that I know, it's entirely unnecessary to discuss further.
ISIS is not the problem in terms of your scenario working, it's the other 2 dozen groups and the effect of your plan in the region and for future US interests.
I don't think you need to be worried about this anymore. ISIS will be defeated in Iraq in 18 months to 3 years and in Syria between 2-5. Afterwards they will be reduced to an insurgency.
I like how you paint me as some sort of non-intervention pacifist when I already have told you what would have worked in terms of Assad regime change. I am all for American involvement as it is today. US strategy is currently working and there are no signs it will not work further.
I think if you read into the SDF a bit more you may realize that you dream may come true. The SDF, if they choose to do so, may very well turn into a Democratic banner holder when it comes to negotiations for peace. The more land the SDF takes and the more cities they liberate the more local support they will receive.
Unfortunately, since they are mainly Kurdish, this will most likely not be gain 90% support of the country. And I think that's part of the reason why you're lost in this hypothetical. The nuance, the factions, the alliances. They all matter, bud. They are real. If you give people a good reason to rally behind a cause then they will, but as the DoD train and equip program showed that reason is not the one you are proposing.