I don't know the particulars of the bill, but as long as candidates are not allowed to claim to represent a major political party that hasn't endorsed them, then this doesn't violate the constitutional right to assembly or weaken political parties in any meaningful way. Far from being squeezed out, third parties could benefit from being able to endorse one of the primary winners. Electoral fusion in New York has allowed minor parties a way around the problem of feeling one's vote is thrown away.
Hardly radical, this is a minor constitutional adjustment. For states that don't have runoffs it would ensure that candidates were elected by an actual majority without the expense of holding a third election. And it could enhance citizen influence on politics, which is a very good thing! More states should be looking to open up the political process -- especially states like Pennsylvania with such convoluted governmental structures.