The Sicilian Defense Kalashnikov Variation: Why shouldn’t white play 5.Nxc6 here? I get a lot of weaker players, who opt out of main lines, and they play 5.Nxc6?! against me. It has seemed to happen a lot lately. Why is that bad, and why shouldn’t white play that?

Note: This is more of a teaching question as I intend to answer my own question.

 [FEN ""]

 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 e5 5. Nxc6?!
First, it is important to know that in many Sicilian positions, not just the Kalashnikov, if black gets in d5, he has equalized, and has a very free game. Thus, taking on c6 helps black reinforce the center, and he will often get in d5 soon. White following up the trade with Bc4, which scores only 20% for white despite white out-rating black 2376 to 2192 per Mega 2020, makes it that much more impactful if black does get in d5 since white will be forced to lose yet another tempo.

In addition, black will soon play Rb8 with a lot of pressure against b2, or if white plays b3, there will be chronic weaknesses on the queenside dark squares, and that could lead to Bb4xc3, and e4 may hang under certain circumstances. Black will also eventually get long-term pressure down both the b- and c-files, combines with a5-a4, should white exchange after an eventual d5 by black. These pawns may look strong, and they would be in the endgame, but in the middlegame, they are just huge targets on open files.

Also, of note is that in most situations, if white plays the natural Bg5, either simply h6 wins the bishop pair, or Qa5 can get out of the pin tactically.

 [FEN ""]

 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 e5 5. Nxc6 bxc6 6. Bc4 (6. Nc3 Nf6 7. Bg5 $6 Rb8 8. Rb1 h6 9. Bxf6 (9. Bh4 $4 Qa5 10. Bxf6 Rxb2 $1 11. Kd2 Bb4 12. Rxb2 Bxc3+ 13. Kc1 Qa3 $19) (9. Bd2 d5 $17) 9... Qxf6 10. Bc4 Bc5 11. O-O O-O 12. Qe2 Bd4 13. Na4 d5 14. exd5 Bf5 15. dxc6 Qxc6 16. Nc3 Rfc8 {With huge pressure for the pawn.}) 6... Nf6 7. Nc3 Bb4 (7... Bc5 $5 {Is also possible, with the idea Bd4.}) 8. O-O O-O 9. Bg5 h6 10. Bh4 g5 11. Bg3 Qe7 12. Qd3 Rd8 13. Rfe1 d6 {And although white is holding it together, it is still not pleasant. The Bg3 is not doing much, the Qd3 is not comfortable being opposed by the Rd8, and black will continue trying to create weaknesses on the queenside with Rb8.}