SBUX is already a luxury brand. Just because their coffee is not particularly good compared to Blue Bottle's, don't forget that SBUX is still charging $3-$4 for a typical cup.
SBUX can't scale Blue Bottle across their distribution channel, which is enormous. So anything they did to introduce the Blue Bottle brand would dilute their own brand.
Meanwhile, a cup of coffee at Blue Bottle might not even be perceptibly more expensive than a cup at SBUX.
Finally, Blue Bottle has minimal brand penetration across the US. You know about them if you live in Williamsburg in NYC, but lots of people in our Manhattan office had never been to Blue Bottle last time I was in town. They're obviously nowhere in Chicago. Are they even in Los Angeles? They're not in Seattle, or Austin, or Atlanta, or Mpls.
An SBUX acquisition of Blue Bottle seems like a mostly-downside, lose-lose proposition.
(I'm only arguing because it's an interesting question, not because I'm sure I'm right).
(Interestingly, Intelligentsia in Chicago has higher revenues and apparently higher year-over-year growth than Blue Bottle, the same number of locations, and no external funding. Maybe SBUX will buy them! Intelligentsia also ran Clover machines before Starbucks' ill-fated takeover).
(Finally: Ritual, in the Mission, is really good; I think better than Blue Bottle.)