I do not believe the first law was intended to assert the existence of inertial frames of reference, else Newton would have said that explicitly. I think you may be extrapolating from Einstein.
More likely, the first law was intended to correct the then widespread Aristotelian conviction that all terrestrial motion eventually ceases.
Newton’s first law is about the existence of inertial frames of reference.
I do not believe the first law was intended to assert the existence of inertial frames of reference, else Newton would have said that explicitly. I think you may be extrapolating from Einstein.
More likely, the first law was intended to correct the then widespread Aristotelian conviction that all terrestrial motion eventually ceases.
However, as a standalone statement it is vacuous.
https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#First