Is it? I've found that all open core projects I've used over time gradually lock more and more features away with each update and usually end up being more expensive than the closed source alternative.
The latest example is RocketChat which a year ago had a couple of features and one or two gentle reminders to upgrade to enterprise, but now it's getting really naggy. And when I did contact them for enterprise pricing, which is not clearly available, it ended up being way more than we could pay and a lot more than Slack (can't remember exactly now but something like $10/user if we included the bridges to WhatsApp and Messenger).
Can you give any counterexamples of popular, growing open core projects that don't lock away more features over time?
If your business is produce software and work as consulting company for it, software that is very good and easy to use is actively reducing your market, because frankly people don't need any help to use it.
Take ES for example. Running big-ish cluster isn't that hard with a bit of googling and experimenting, so there is little reason to ever bother with support contract.
We had exact same experience with Mattermost. We needed only LDAP auth from enterprise side but it was good product so we were fine paying iirc $2.5/mo/user for it. They doubled the price ("we add more features that you don't use to this level, so pay us more"), we got pissed and our dev just whipped github-like auth page (OC version allows authenticating with gitlab) and we pointed it to that.
Now I see they doubled it again and at $10, when our O365 accounts cost $6.50/mo it would be impossible sell
And you don't need open source model if you just want to give smaller might-be-a-customer a free, limited version.
I guess it depends on what exactly is behind the paywall. Some have very weird limits, like Mattermost where you can use Gitlab for authorization in open core but for LDAP you need enterprise.
We actually paid for it and were pretty happy with it but at some point they decided to double the price under excuse "but we added features"(that we did not use) to that version. So one Rails dev added a dummy "gitlab" oauth page for auth that's connected to our LDAP and we just switched to open core version...
...aand since then they doubled the price again. So suffice to say I'm a bit soured on open core model, when they tried to upsell us to more than we pay for O365 account for same features.
I'm not saying it's all bad, for example VictoriaMetrics looks like it have very reasonable model (although lack of pricing on their page is... annoying), but it is model ripe for abuse, especially if company gets bought.