Battle of Thermopylae was a loss, not success.
https://en.wikipedia.org/wiki/Battle_of_Thermopylae
> Result Persian victory[a]
>Territorial changes Persians gain control of Phocis, Boeotia, and Attica[5]
In the Thermopylae case, they failed at their intended objective (defeat the Persian army on land) and wasted a good-sized force. The fact that the defeat was set in stone the moment they chose the place and time of the engagement doesn't make it any less of a defeat.
(Note that "the 300" is only counting the Spartiates i.e. citizens; there were a total of 1-2K Spartan soldiers, and many more forces from other Greek poleis.)
Anyway, even if you grant that they were just trying to buy time (they weren’t), dying to the man in exchange for an extra day or so accomplished nothing militarily.
The battle of Salamis where the Greek defeated the Persians was definitely considered a success at the time, not a failure because they didn't get to die heroically every last man!
But in case of a defeat it is common to spin it as some kind of moral victory. Just see how the British spun the defeat at Dunkerque in the beginning of WWII. But that does not mean they didn't care about winning.
The Greeks also had very different rules awarding time for sea combat and land combat (rooted in Homer). In land combat, the model was Achilles, and stratagems and tricks detracted from the time of a victory, while in sea combat, the model was Odysseus, and metis ruled, so strategems and tricks were in bounds, hence the glory of Themistocles.
ie people who want to reproduce the mythical Sparta often advocate the social structure of the real Sparta, thinking it will lead to those mythical benefits. And instead it leads to monstrosity.
What I think is going on is not that people like what Spartans actually did, people are simply enamoured by their honourable and majestic portrayal in popular culture. What youngster does not fantasise of being that small and determined band of soldiers that holds off the hordes of brutal barbarians, only with their shields, spears, courage and martial prowess? Who doesn't want to be part of that 250 men that defy a whole empire, _and succeed_, immortalised in songs and poems? (I mean they all die by the end, but they do kinda succeed in their mission).
The way I think about it is that for me there are two (or more even) Spartas. The historical "Sparta" that were brutal oppressors, like a case study how _not_ to structure a society, lots of lessons to learn from it. And the mythical "SPARTA!" one, full of honourable dudes and courageous women, that we can aspire to be, even if they were not real. No reason why they can't coexist in our minds, we just need to be careful which one we talk about.
When people say they "like Sparta" I don't immediately feel that they are silently singling that they will be ok with slavery and secret police, I think they want to be strong people fighting to protect their brethren, and that's generally OK with me.