The whole thing was poor writing.
In season 2 episode 1. When we first meet Dr. Polaski she is quoted saying to Data: “I’m sorry Data, Starfleet has determined you are a sentient being. And I’m just going to have to learn to accept that”.
And then just a few episodes later in the same season we get “Measure of Man”.
And then even after MoM Picard threatens Data that Starfleet will strip Data down to his wires bc they think he is lying to them.
Starfleet has some really messed up philosophies.
They can be, yeah, but more often than not they come out in the right place.
In any event, I think Data’s sentience would’ve been established when he first enlisted, or when he entered Starfleet Academy. Bit late to treating it as an open question when he’s already a lieutenant commander.
In ST:TNG ep. “Booby Trap” Picard and Riker are trapped in a toxic asteroid field and mulled over turning navigational control over to the Computer.
Picard admitting the computer can take orders, but wondered how well it could give them.
In a ST:VOY (ep. “The Fight” ?) the computer successfully warps out of the difficult-to-navigate space Voyager was in.
So, a Starfleet Computer could be…and was… successfully put in charge.
I meant more like M5 being in command. Choosing destinations, picking away teams, ordering who gets latrine detail this week, that sort of thing. 
Writers write for dramatic impact not for really for proof of law, situations are merely used (and thrown) to support drama (which is the goal itself). I love Legal Eagal’s starting clip which highlights how weak and vain Maddox’s arguments are (which only serves to rile the viewers up).
Better writers might’ve had Picard ending his speech both powerfully and tackling consciousness in a short and effective matter (for the viewing audience)… but it was a pretty emotional end to keep the speech quizzical and confrontationally posing the question to all doubters in the court.
Well, yeah; but the unfair advantage is, we know how the episode ends, and the writer knew how the judge would rule in the wake of Picard’s speech.
That is, we’re not in — and never really were in — a situation where Picard could lose, and later have someone ask, shit, Maddox named three criteria, and in two minutes and change you established that He’s Met Two Of Your Three Criteria; and then you didn’t make a case for the third, and then the judge ruled against you? Like, you said that two parts weren’t at issue, and Maddox didn’t really argue either point; and, as an expert witness, he testified that he’d concluded that Data didn’t pass the three-part test, and you didn’t really argue Part Three; and then the judge said she had no choice but to rule that Maddox’s conclusion went undisputed and so had to carry the day by default? So you made it clear that you’d get all up in his face about One and Two, and then you made it equally clear that you had nothing on Three? What the hell did you think would happen?
Yeah, being TNG’s breakout character and viewer favourite, Data wasn’t in any effective danger (it would be a great twist to have Picard fail, but the story must wrap-up).
A better technique is to ingratiate side/minor characters to the audiences and then threaten them with peril. Like what was done to Lal (Data’s daughter) when a nakedly dismissive Admiral arrives to remove her from Data. Picard raises the legally recognized rights androids (now) have and fights when he sees that these rights are clearly not being adhered to.
If you want o give real peril it must be directed at killable characters. TNG rarely did this perfect and often introduced new characters just to be slaughtered too quickly (introducing and then fridging was standard in TOS). In the end all this juicy conflict was quickly removed by having Lal die due to random technobabble. Pity, but I understand; the story must wrap-up.
In review, I wish I wrote my above tangent in a more spoil friendly manner. Apologies folks.
It is, of course, a typical TNG Aesop ep. not the best, not the worst, but I’ve always thought that, legally, Picard took the wrong tack.
Data has already been granted the civil rights of a sentient being; he’d been accepted as a cadet, graduated and commissioned as a Starfleet officer, you can’t do that and then say ‘backsies, you’re a toaster.’ That’s like Nuremberg laws evil.
If war broke out with the Klingons would they arrest Worf for treason or espionage for having been on the bridge of a Galaxy class starship, or would they offer him a post on the far side of the Federation, or civilian status under observation?
True, the language of Federation citizenship laws my use words like ‘born’ where ‘created’ might be more inclusive, but that’s small change compared to the precedent of data’s long-standing self-determination as an officer.
Or maybe just kill him out of hand. But who knows what a well-motivated Klingon officer might do?: https://www.youtube.com/watch?v=OKQDRxs5Nzk