If you ever get something for free, remember something somewhere is subsidizing it. Whether that's the other paid accounts, which therefore must be uncompetitively priced, VC funding (which will eventually run out), or your data is sold.
Our experience went like this: We got Bitbucket for out project for the good free private repos (and a few other reasons). When it came to choosing a ticket system, being on Bitbucket made choosing Jira a no-brainer (one of the best ticket systems, good integration with Bitbucket, familiar interface). Then we needed a better wiki for internal documentation, so we naturally went with Confluence.
So getting free Bitbucket heavily influenced our decision to buy two other products from the same company, and we are about to buy into their CI system (Bitbucket Pipelines, not the other one). We don't regret any of those choices, but they might have looked different if we were on Github.
It's now easier to see some of this, with the GDPR rules.
The Gitlab cookie page lists 80 "marketing" cookies which may be set: https://about.gitlab.com/privacy/cookies/
If you don't see this, it could be informative to use a British or Irish VPN (to keep an English language default) for a day or two.
Some is, eg some linux/gnome devs are paid to contribute. But many smaller projects/components are volunteer-only.
Someone is paying for it with their spare time. Spare time doesn't last forever.
If you're not paying for something, you should see it either as a temporary shortcut that eventually needs a more sustainable fix, or something you don't care about if it disappears.
Many many many many many commercial software projects, that people paid money for, are killed every single year.. Many other volunteer only projects last for decades, some of which have been critical to the very foundation of the web.
So no "if you are not paying for you should see it as a temporary shortcut" is a completely false statement
Granted, there's nothing nefarious about this, but there isn't anything necessarily nefarious about the fact that anything you get for free is being paid for somewhere else. Just potentially nefarious, if you don't know what's paying for it.
That doesn't follow. If the paid accounts are selling, then they must be priced appropriately relative to their competition and the value they provide. The cost doesn't factor into it - and free accounts are just a marketing cost, like putting up a billboard.