I get the meaning of the phrase back when Ben Franklin allegedly said it, but what does it mean today?
I believe it can be summed up as “History is written by the victors.” Please note, not said first by Churchill.
I also believe you’re seeking opinions here. So let me move this to In My Humble Opinion from Factual Questions
I am, thanks. I saw it on a shirt and wanted more information before speaking to them and getting drawn into a discussion.
Treason is opposition to the established government.
If you have a revolution and win, you establish a new government. The people who were traitors to the old government are now loyalists to the new government.
We’ve also seen the opposite claim: The January 6 insurrection wasn’t really an insurrection, because it didn’t succeed.
As to why someone would put that on a T-shirt, my first thought would be that they’re saying that they have to make sure not to fail this time, in other words, that they’re currently plotting treason. Actually, that’s my second and third thought, too. Even if one considers the statement to be true, I can’t think of any other reason one would want to put it on a shirt.
I tried googling the shirt and got a few Libertarian and far-right places selling it. Shows how much I didn’t know this person. 
I think it would be more accurate to say that the January 6 insurrection wasn’t really an insurrection because the people who did it don’t know what the word insurrection means.
Sure they do. You get a bunch of roads all meeting at the same place with a light. That’s an insurrection, right?
I’ve seen January 6th more described as the attempted insurrection. Which you can still be tried for same as attempted murder.
The winners make the rules.
Of Treason
John Harington
Treason doth never prosper; what’s the reason?
For if it prosper, none dare call it treason.
So just based on the shirt- right-wing or no? I realize it’s shallow to judge just based on the shirt.
It’s possible that the shirt wearer doesn’t know what it means.
99% chance it means they’re hard-right; dangerously hard right. The shirt may well have said “I support armed revolution”. 1% chance they don’t know what it means and they think it’s profound somehow. But even that is pretty far-fetched in today’s world.
On a semi-related note, last night I attended a baseball game. On the 3rd of July and billed as a tribute to veterans. With fireworks after the show. As you might imagine, performative patriotism was much on display by the crowd. As well as a lot of old codgers in VFW regalia or wearing veteran baseball caps.
Both I and the guy I went with are veterans, although we have no truck with the right at all, much less the hard right. Hard-rightness is not patriotism no matter how hard they try to wrap themselves in the flag that represents what they actually hate.
Anyhow …
I saw a guy wearing a T-shirt saying “If you live in this country support it. Otherwise move to the country you support” Which of course is a long-winded way of saying the same idea as the Viet Nam era slogan “America: love it or leave it.” Which also means “Unquestioningly obey your superiors in government. As long as they’re the right superiors.”
Which T-shirt got me to thinking of a corresponding LW idea.
I came up with “If you love your country, work every day to improve it. Not vandalize its improvements.”
In each case there’s an assumption by the speaker of what “support”, “improve”, or “vandalize” means. But I think it gets the point across. Especially if there’s some other overt progressive / LW / Democratic cue.
I mean, that could mean that Trump, Marjorie Taylor Greene, etc. should move to Russia. That’s probably not what it does mean, but it could.
This thread reminds me of a key exchange in Clavell’s Shogun:
'“There are no ‘mitigating circumstances’ when it comes to rebellion against a sovereign lord,” chided Lord Toranaga.
“Unless you win,” John Blackthorne replied.’
We only celebrate Washington and the American founders today as heroes because they won.
If they’d lost, then the government would have remained British, and the revolutionaries would have been treated the way we treat the Confederacy today.
“Treason doth never prosper: what ’s the reason?
Why, if it prosper, none dare call it treason.”
― John Harrington
It’s awfully echoey here in these big old library buildings full of dusty quotes. 
That’s what happens when a young female cat demands attention while you are trying to read a thread.