What specifically would a user notice with the difference between Blaze and React? I get react is a bit faster under the hood. Its not something that would make this a 'serious' alternative?
I'm an avid react user and I completely agree. It wouldn't make it a 'serious' alternative just from that, although I would always love to see react used :).
The serious alternative is having both: API integrations + react, specially the first one... Like you say, react alone won't make a big difference, but it could potentially make the UI less laggy.