I wonder if PMM is also a management tool or just a pure monitoring tool. All what I’ve read and seen so far of PMM is about monitoring but its name PMM, the second M, suggest more than just monitoring.
I watched also “Run your own open source alternative to Ops-Manager (MMS)” by David Murphy. He said at the time (more than 4 years ago) that the management part was not yet implemented but was on the to-do list. Has it become more than that in the meantime? If so, to what extent can you set up and manage your MongoDB, MySQL and Postgres environments automatically - as Ops-Manager (MMS) already does?
As of right now PMM is mostly (well, entirely) monitoring but one of the big hurdles to building in the management functionality was PMM1, it just wouldn’t have been pretty to force it in. We launched PMM2 in Oct of '19 and have been iterating on making sure we have close to parity between the two products while at the same time getting more of our infrastructure in place to support not just getting data but being able to do something about it…SAFELY! This has obviously slowed down our pace a bit. We have, however, set a few foundational elements in place already with more to come; our Security Threat Tool release (in 2.6.0 and later) will likely be the first to leverage management capabilities which can detect issues now and will soon give the option to fix them (think of: Monitoring - Detected user with blank password on a mysql db. Management - Click “Fix”, provide elevated credentials for system issue was detected on and password you’d like to use and you can safely and securely make that update from the PMM UI).
As for the second part of your question, Ops Manager is still the gold standard for Mongo…We’re developing a Database as a Service product right now as part of PMM to allow you to be able to automate more of your administration tasks across the various database environments centrally and consistently (via UI and API). And this is not something that just the PMM Platform team is working on but other groups as well are actively developing product to work in concert with PMM. So if the need is immediate, there’s not much today but if you’re starting the investigation process for Open Source options, it won’t be long before we start to emerge as one.
Thank you Steve for the detailed answer! Yes, Ops Manager is still the gold standard for mongo - a gold credit card is unfortunately required as well 
One more question: Will the new Database as a service also be usable for on-prem servers?