Because we already are and he'll want more? He was one of the ONLY backups with 2-3 starts to not be able to surpass his starter in a wide variety of metrics: Completion percentage, success rate, passer rating, QBR, ANY/A, sack rate... he did worse than Geno in all of those.
Most backups have at least one thing they do that they pass their starter on just due differences in their style or the way defenses adjust to playing a backup.
Drew did not.
Geno is playing like a top 15 QB. He has a top 15 QB contract. He's not playing like a top 10 or top 5 QB (this year) but he doesn't have a top 10 or top 5 QB contract.
Drew played like a bottom 5 QB2. He has a top 5 QB2 contract. That's the definition of overpaying.