Would you be upset if people came into your house uninvited?
Nobody’s proposing that. The California state constitution is pretty clear that beaches are public access. Khosla more than most knew that going in, well before he was in a position to grab his rake and run up and down the beach yelling at sunbathers. This wasn’t a shocker. He wanted to litigate this case well before the ink was dry on the purchase papers, and kicked it off arguing the Treaty of Guadalupe-Hidalgo that ended the Mexican-American war superseded the state constitution. Fun fact: it did not.
Other jurisdictions like Australia have public access to beach rules. In much of Europe the same rules apply to much more inland areas too. And you know what? It works great there, and surprisingly, you can’t sleep with the spouse or anyone who owns beachfront property there either.
Your slippery slope is flat and dry.
Walking through someone's yard isn't a good comparison wither because a medium plot of land is maybe 4 thousand square feet and his property is 4 million square feet.
We had it hammered into us from childhood, and it boiled down to getting us to think about what would affect us if people wanted to access our land, and what would have only minor effect, and what kind of access we might want to have that would have at most minor affect on the owner. It takes minimal prompting to get kids to come up with trade-offs that deals with the distinction of "walking through someone's yard" vs. "grant access to larger areas of land".