Nucleotides are hard to make. 5' phosphates hard to make. GC base pairing and AU base pairing is simple and plausible Problem lies in the backbone. Fidelity is very important. If next-generation biopolymers are unable to facilitate self replication the advantage is lost.
Variation is important.
Best system would have had a balance between fidelity and variation. Thus a primitive replicase or polymerase was likely to be one of the first biological activities. The selfish gene Transposons, plasmids, bacteriophages, viruses. Most likely evolved at various times directly from the genome through physical dissociation of the gene from the chromosome. Extrachromosomal genes could replicate faster than chromosomal genes.