I suspect the true answer is that sentience is defined as a spectrum of phenomena ranging from matter that can make some sort of a measurement (like a thermometer) to simple “agents” that have distinct boundaries and a mechanism for controlling that boundary, all the way up to systems like the brain - and perhaps far beyond.
That definition simply doesn’t help you tackle ethical issues in the way you’re presuming- where we can say “this is sentient, that is not”. But it is a possibility.
I would say it already does to some degree. We choose to not eat certain animals like dolphins, but happily eat fish. Society determined in very informal terms that they are more sentient than fish and therefore they should not be eaten.
I would also like to say that I made the 'line in the sand' point for arguments sake, and that I personally think it is a spectrum.
That definition simply doesn’t help you tackle ethical issues in the way you’re presuming- where we can say “this is sentient, that is not”. But it is a possibility.