From the Reds perspective, Senzel as a RF with an .800 to .850 OPS is much more valuable than as a 2B or 3B at AAA. I just don't see him being a big leaguer in 2018 if he stays in the IF. He nor any of the current projected Reds at 2B or 3B will be moving to SS to open an IF spot. Gennett won't add much to any trade return and I think it's doubtful they'll deal him. They could get the same, maybe better, by dealing Schebler or Duvall and Senzel would be a bigger upgrade in place of either of those guys than he would replacing Gennett..
I also dispute the widely held assumption that the transition from 3B to 2B will be all that easy. Todd Frazier couldn't do it. Aaron Boone couldn't do it. Playing the OF is much easier than playing any position in the IF (including 1B IMO) provided that the player has the basic tools required to play in the OF. Senzel has both the mobility and the arm strength to make a very solid defensive RF. He's not Yonder Alonso or Adam Dunn. All that's left is a short trial period to see if he can read a fly ball off the bat. If he can, he could probably move to the OF fairly quickly without much problem like Duvall and Betts did. If he can't, he'd probably always be poor out there no matter how much work he puts into it, and I would abandon the idea. IMO, it's his quickest path to the big leagues and getting to the big leagues quickly is a much bigger factor in how much he'll earn over his career than the position he plays might be. A transition to 2b will likely require a full season at AAA learning the position. If he becomes a free agent before he turns 30, the deal he can sign will be significantly higher than if he reaches that level at 31 or so. The principle that says he's more valuable as a 2B than a RF is true in a vacuum, but nothing is ever done in a vacuum and reaching the big leagues at a younger age is a way bigger factor on how much money he'll earn in his career. It's also better for the Reds and moving the rebuild forward. Senzel will be a building block no matter where he plays. Lets get him up here.