I;'m assuming you mean in OT? If so that's been discussed to death today in the bay area. Shanahan said he took the ball because the defense was exhausted. Between the injuries and the fatigue factor he decided to put his offense out there first so his defense could rest. He was well aware that Mahomes was going to get the ball no matter what.
My big complaint would be given the situation I think they should have gone for it rather than kicking the FG. I think they got "spooked" by 2 plays, the play before it where CMC got stuffed for no gain, and the play a few plays earlier when Mitchell went around right end but it got called back for a hold on the backup TE. The feeling is Shanahan was afraid if they ran it on 3rd down and didn't get it, they'd have to kick the FG, so he threw the ball on 3rd down but it was incomplete due to pass rush so he had to kick it. Had he ran the ball on 3rd down and gained yards he may have had more choice on 4th.
Interestingly apparently the coaches were furious with Mitchell, not the TE for the hold. Despite the look of the play, it was apparently NOT designed to go outside but Mitchell broke it out and then the TE grabbed and held trying to help him get the corner. Apparently the coaches said the entire penalty is Mitchell's fault for trying to go wide.