That makes sense. Would be a big change of habit for me, since by phone tends to stay in my jeans pocket while I drive. Fishing it out while seated in order to plug it in would be a (minor) hassle.
All in all, I think key fobs are a bit like television remote controls. Ultimately, most folks prefer the ease of having a dedicated, 100% reliable, device. Given all the problems Telsa has had in the past with novel door operations, I can't believe they deviated here from the industry-standard of providing a fob.