the standard model isn't one thing, it's the sum total of human knowledge of particle physics. it's an equation with a gazillion terms - this is an adjustment to one of those terms. and yes, you can add terms for any new physics you discover, so technically you're right, but it's not a gotcha.
This makes it sound more ad hoc than it is, it's not some polynomial where people just tack on terms.
In its current agreed upon form it's just SU(3)xSU(2)xU(1). This gauge symmetry defines the lagrangian, which has 19 parameters to be determined by experiment.
It's true that this isn't the whole story (dark matter etc), but these symmetries are physically motivated and their predictive power is pretty amazing (the QED part is CORRECT as far as any experiment has been able to check so far).
thank you! one day I'll understand this stuff - I skipped the qft elective at uni, but I'm trying to learn more now.
'The standard model' also doesn't include gravity. And we do have pretty good theories about gravity.
So it's definitely not the 'sum total of human knowledge'.