I'm not going to get engaged in the whole Schoch controversy regarding supposed water erosion patterns on the Sphinx (though I note that Schoch and his qualifications are being discussed elsewhere on AH in another thread at present.) But, while you're waiting for fresh responses to your query, you could check out an earlier response of mine which discussed whether and in what circumstances the Egyptians of the Pharaonic period kept ancient monuments in repair. The tl;dr is that upkeep efforts were focused on religious ceremonial rather than things such as cleaning and repairs.