It's far far harder for a faction to defend itself when their enemies have massive air support and they do not.

Isis lose with or without boots on the ground if their current enemies in the region are simply given heavy air support and supplies.

Putting boots on the ground, Benefits:

More local control.

Better trained soldiers.

Cons:

Expense cash.

Expense lives.

Needing to run an occupation.

Difficulty of nation building due to resentment against your new state from local peoples about foreign occupiers.

Has anyone ever won a war with air power alone? Arguably the 1999 bombing campaign of Serbia - but then you could argue that that is notable specifically as an exception, and had some special circumstances surrounding it which don't seem to be present with IS. See https://en.wikipedia.org/wiki/NATO_bombing_of_Yugoslavia#Arguments_for_strategic_air_power

most folks who have looked into this matter with any kind of effort/depth argue that putting boots on the ground is pretty much the only way of successfully challenging daesh.

Who are these "most folks", what are their incentives, and what alternatives have they considered?

I would say that a ground campaign would defeat IS in Syria and Iraq. I cannot be bothered to dig for a definite source for that, but USA, EU military is vastly better than IS and I don't want to waste my time arguing with anyone who thinks that an America/EU/Russian alliance would lose a ground war against IS in Iraq and Syria.

Then why didn't the US win against the groups that are precursors to daesh when they had boots on the ground in Iraq?

Yes US/EU troops are superior but as afghanistan and iraq showed, simply having those troops in the country does not solve your problems. Military victory, in the sense that a nominally friendly government ends up in control can be achieved without boots on the ground, it's far easier for friendly groups to win with air support and supplies but you have to avoid public perception that the new state is just a puppet.