Users of these projects should expect something like the Skiff sell to happen -- especially if the project is open core or does not use a truly permissive license.
The execution needs to be carefully balanced on all fronts, which many companies don't do correctly giving this business model a bad reputation, but I wouldn't refer to it as "always a marketing tactic", or as something inherently wrong with the open core model.
I'd go as far as to say that open core is possibly the best way to monetize open source projects.
For me, user freedom is guaranteed by free software licenses.
The thing you sell also don't need to be proprietary. I work for an open source company that sells free software [1], support, hosting and consultancy. It specifically rejects doing open core in our business decisions. I'm glad it has worked so far (and has been for 20 years this year).
(coincidentally, CryptPad is one thing the company makes :-))
When the open core model is implemented correctly, the "core" part of the product should be an entirely standalone piece of software, that aims to be best in its class of products. This is already a win for users. The proprietary components around it should be value-added features that are not critical for the core functionality. E.g. optional plugins, enterprise features and services, etc.
When this is done correctly, commercial users effectively subsidize the free software for everyone else. This is a good thing.
Trying to force companies to be fully open source, and shaming them otherwise, hurts the adoption and spread of free software in the long run. Running a successful company is difficult enough, and running a successful company that builds OSS even more so, so companies that do this right deserve to be praised. Those that don't should obviously be shamed, but the fact that some don't doesn't make this business model wrong. They're just shitty companies.
I agree. Read my other comments. My company achieves this without open core.
When you believe that free software is the right thing to do, it's only logical to think that commercial users should also be able to enjoy the freedoms of free software.
Free software or not, companies will pay for enterprise features if done right.
Open core is better than nothing but it incentivizes putting interesting / killer features outside the core. And you live from selling proprietary software, that sends a mixed message. "We believe in open source, but [you can't live from it|we don't believe you need your freedoms]"
When you are able to sell free software, any feature you provide is open source, you can sleep on both your ears and the message is strong: "we believe in free software and we respect your rights. We live from it and if you do business with you, that's what you get". There's no tension between the open core and the outside. Other people can contribute to your paid part too and that's amazing (yes, we have this).
Open core is not the only / best way to implement commercial features.