Despite what Sinatra would have us believe, if you can make it in New York it doesn't mean you'll make it in the mid west or any of the purple states. (Democrats haven't had a vote share lower than 65% since the 00s I think)
Yes, a record number came out to support him but almost as many came out to support anyone but him :(
My hope is that for all the naysaying, Mamdani turns out to be a technocrat in the Sewer Socialist model and shows the country socialism ain't so scary. The whole "laboratories of democracy" in action.
The argument has historically been that the Democrats don't nominate progressive candidates and if they did, progressives would come out and vote for them overwhelmingly (despite not doing so in the primaries.)
In one of the most progressive cities, we had one of the most progressive candidates ever and barely cracked 50%.
So, it doesn't bode well for the Dems chances if they nominate a very progressive Presidential candidate. (You would probably have the Blue vote similarly, wherein sure, some progressive would win the Democrat label, and some independent would run to the centre and split the Dem vote.