Developers take on the new responsibilities of being able to independently deploy their code, instrument and monitor stability and own test/QA.
Platform Engineering is about building a robust infrastructure and the tooling needed for Developers to handle the new responsibilities. This includes packaging, monitoring, deployment, AB testing, etc.
Site Reliability Engineering is about dealing with fires outside of the codebase. Hardware failures, network connectivity issues, etc.
I don't think any of these roles becomes a "Jack of all trades, master at none" situation. It does, however, cut out some of the more typical engineering roles. While developers just took on additional responsibilities, QA engineers and traditional Ops are forced to repurpose their skill set.