They do not need to make any technical changes. They could make the change overnight: $1 per repo per month (or whatever).
This would stop the aggressive growth that they clearly cannot handle anyway.
The question is: are they willing to compromise growth for QoS. So far the answer is a resounding: "no". They are deliberately sacrificing the experience of existing users to gain new ones.
No way I pay $1/repo/month. No way I pay $1/repo/year. I still host and maintain my old code on GitHub, but any free tier pricing update and I jump ship immediately.
There are tons of existing free users, they'd be sacrificing those too. And yeah they don't want to sacrifice growth either, so the remaining is an engineering question.
The question is: are they willing to compromise growth for QoS. So far the answer is a resounding: "no". They are deliberately sacrificing the experience of existing users to gain new ones.