Making space for implants between upper and lower teeth

[Brook]

I am in need of some info. I recently had a complete sinus augmentation in preparation to have implants placed in space 13 and 14. I was informed by the dentist that there is not enough space to place the implant between upper and lowers. He told me I would need to have a root canal on teeth #18,19 so he can grind them down to make room for implant.
Are there any other options?
Why couldn't he have made room for teeth during the sinus augmentation?
Any help would be great as I have never heard of this before. Thanks!