Not every opensource monetization scheme is the same. Mongo and Gitlab are not the same. One is mission-critical production software, the other is a productivity tool (which can become critical, but in a different manner), thus different ways to monetize. Hashicorp, Canonical, Mesosphere and NodeBB are not nearly the same. Why put everyone on the same pot?
Opensource is not an industry. Opensource is a guarantee, a certificate of transparency. Opensource is global-scale collaboration. And much more.
Neither is everyone moving everything to fully-managed PaaS, at least not in the way described by the OP, who assumes a lot of givens there. Many use cloud infrastructure, but not all the services, some have private clouds or other hybrid setups.
Application platforms are constantly changing, why now assume Mongo or any other opensource models are doomed? MongoDB Inc. is a public company and is perfectly capable of defending their bottom line against a magnificently sized but over-committed competitor like AWS. Mongo has NOT done a good job with their PaaS or enterprise tooling so far, competitors have popped up, so now they have to up their game, that's all.
To underestimate OSS monetization is very presumptuous. To compare musical recordings, an end to itself, to infrastructure mission-critical software is downright stupid.