It's not necessarily a case of "LIDAR is necessary": it's more a case of how much more difficult it is to extract the necessary information with the necessary reliability from cameras alone (one important aspect I think is that LIDAR + HDmaps is very reliable at telling you 'something is there', even if it's not able to distinguish details. From the point of view of the car operating safely, that's a big deal). As it stands Tesla is behind waymo and making things harder for themselves (fundamentally because they've been trying to make it a consumer product, which constrains costs much more than aiming for robotaxis). It might mean they eventually get a much more cost competitive product, but that won't matter if they're beaten to market by 5-10 years (at the moment you can argue tesla is ahead in the areas their driver assist can operate and number of vehicles, but I think it's going to be much easier for waymo to scale up and to different areas than it is for tesla to progress their tech).