Don't be so sure of that. Democrat/Republican state legislatures have a knack for changing the ballot access rules once some third party manages to achieve "Permanent Ballot Access". Saw this happen more than once in Illinois, petitioning for ballot access on behalf of LP.
It’s state law. It’s been the law for decades. Right now, New York has 8 ballot-qualified parties: Democrat, Republican, Conservative, Green, Independence, Working Families, Women’s Equality, and Reform.
If you get 50,000 votes for governor, you’re on. Period.