I'm not sad about it, turned out really well for me.
I believe the compelling part of the current offering is the schema management features. We started building these when I was there. I don't know to what degree the alerting part of the APM product survived. It was originally requested by an enterprise customer and I re-implemented it on-top of Druid to make it much more scalable. My assumption is that should be a bit of a draw. etc.
Nice. Relatedly, as someone who has a few years of experience, how does one become a principal engineer? Anything I should know or do to get to that level?
I think most important aspect is experience, the second most important would be leadership. There is little point being a principal engineer if the intuition and experience you can bring to bear aren't embraced by your team so the soft skills definitely count. The role generally encompasses tasks that go beyond writing code, architecture being the most crucial (and technical) but you will also be expected to take part in hiring, making strategical and tactical decisions, etc.
Generally you get there through a senior or staff engineering role where you can demonstrate the technical and leadership aspects. I would focus on mentoring your more junior engineers and taking part in force multiplying efforts, i.e tooling, processes etc.
To be honest I don't understand Apollo's business model, not once did I feel like I needed to pay for it, its free version solves all my needs.