Caruana vs Carlsen game 10 (WCC) why not 18...Nxb6? In game 10 of the Caruana-Carlsen WCC, why did Carlsen (black) not take the terribly annoying and strong dark squared bishop on b6 on move 18 when given the chance? Instead, Carlsen moved the queen with Qe8.

The position is shown in the following diagram:

 [title "Caruana vs Carlsen, game 10, why not 18...Nxb6 instead of 18...Qe8"]
 [fen "r1bq1rk1/1p1nb1pp/1B1p4/P2Pp3/2N2p2/8/2P1BPPP/R2Q1RK1 b - - 0 1"]
From a purely positional point of view, 18...Nxb6 would have been a
terrible mistake, as it would have allowed white to then forcefully trade the light squared bishops with the immediate Bg4
move, leaving exposed all the light square weaknesses that black has
created with the d6-e5-f4 setup.

These weaknesses (created holes) would then have been permanently exploited by white's
knight (and queen), in other words, white's minor piece (b6 knight) would have
dominated the game and been virtually untouchable on light squares for the rest of the game.
Notice that next to the knight controlling key light squares from b6,
it's also for example quite feasible for white to re-route the knight
to e4, established centrally (permanently) and constantly eying black's
weak d6 pawn.

And as for black, Carlsen would have simply been left with a bad dark
squared bishop hemmed in by his own structure.
Therefore, Carlsen chose wisely to play Qe8 instead of Nxb6, thus,
preserving his c8 bishop as Bg4 no longer forces a trade thanks to
the well placed knight on d7.

In summary, this is a purely positional decision where black is well
aware that having weakened their light squares, any scenario with the
knight vs dark square bishop would be unfavorable for them, and
arguably,
at that level even objectively lost! Instead having kept both the d7
knight and c8 bishop, black has in fact preserved two pieces that can
provide light square control.

The following situations would have ensued had Carlsen played
18...Nxb6:

18...Nxb6 19.Nxb6 Ra7 20.Bg4



which would have led to positions of the following nature, infested with light square holes: