Paying for software is one thing. Paying for software where the amount you pay may be all but completely dictated by the supplier is another. If you decided to pay for a relational database, you could, with some work, swap vendors. If you paid for datomic, you could not. That makes it a non-starter imo as a technology decision. The business model here is naive and will change or datomic will always be super-niche. Maybe that's be design, but it's a little sad to me as it sounds like the tech could do some real good.

edit: typo

Lot’s of companies pay for software. E.g. Windows Server and SQL Server.

Of course they do. I'm criticising those companies that claim that they cannot use certain products because they're not open source, while not open sourcing their own. It is just an oxymoron for not reaching to the wallet.

Blame it on decades old experienced marketing and salesmen department at Microsoft. I'm not sure Cognitect has any firepower here.