Yeah the game doesn’t fall apart until after 15th but you can see the stress on it as early as 7th or 8th and 9th level full casters already break away from the mold of most fantasy casters.I have no problem removing 7th-9th spells for PCs as a variant (or even as a default rule).
0D&D spells , originally, went to 6th level (I think Clerics originally went to 5th). Higher level spells were added in later supplements. However, some of Gary's own players have stated that his players never acquired 7th-9th level spells and, one player stated that those spells were not meant for PCs- being intended for NPCs and included for completion sake.*
*According to DM David, Gary wrote that he designed the game for PCs to stop at 16th level and never for them to acquire 9th level spells.