It's brilliant promotion, all the way around. Every few weeks this year, one side or the other drops a nugget and chums the waters, every time the mainstream sports media (and most niche combat sports media) lap it up. Because Mayweather = clicks, McGregor = clicks. And for every honest/legit sports journalism site that calls BS, there are several who are completely complicit in the con, including the Mothership. I don't know much about the MMA side of the street, but there are some legit boxing matches that could use 1/10th of this build-up.
I would bet heavily that it's going to happen. It's light work for Mayweather, and it's very likely a nine-figure payday for doing so. Secondarily, he gets to an even 50-0, I guess. But all indications are that he's serious about it, as he should be for the kind of money that it's going to create. And for as much money as Mayweather is sitting on, his burn rate has to be incredibly high, as his love for depreciating assets, things like cars, watches, etc.
He also, apparently, got swindled this year for a ####-ton of money. This also happened in 2007, when some con artist cleaned him out.
The fight itself is hard to cap, as it's hard to know what shape his hands are in, and if he's going to be willing to step on the gas and get McGregor out of there. He doesn't appear to take any extra satisfaction from a stoppage, and he's barely ever knocked anyone out since he moved to welterweight/super-welterweight. I think the odds could get absurdly short on him very close to fight time, which is when it'd be ideal to pounce. Just, please, don't pay for this garbage.