It being the first in a series, and it being free, doesn't make the entire series free...
And I just checked the
D&D Direct, the relevant part starts at
48:48. They say the series will continue to be updated (
49:40) and throughout the year there will be more (
49:55). At no point do they clearly say it'll all remain free, only that it'll be available. (The video does earlier confirm the free Spelljammer starter adventure, FWIW.)
As for the after-show, if the only record we have is what's on third-party news sites, as mentioned they're also not consistent about whether or not all the installments will be free, or just this one.
I'm not saying they definitely won't be free. I'm just saying there's no definitive statement, which leaves room for future installments to cost money. And it wouldn't surprise me if that's exactly what they do.