I actually don't necessarily think that's true - As someone that has a bit of a background with the codebase of Omnivore I think the thing that killed that wasn't necessarily the business model (let's be real, they didn't even offer a premium tier.)
I think it was the introduction of features that required an unnecessary amount of processing power. Namely, RSS feeds. Their RSS implementation parsed every new webpage - a large percentage of which would never actually be read.
They hosted on Google Cloud using things like Cloud Functions. A good proportion of articles were parsed using Puppeteer, when a cheaper shorter running HTTP Request would have sufficed. The PDF viewer they used cost an arm and a leg.
None of this is to shit on the legacy of Omnivore, because I think with the team they had they built an incredible product. But I think there was a lot that could have been done to reduce monthly costs, and that there could have been more effort to monetise.
I paid for Pocket (without using premium features), and I donated to Omnivore, but the thing is ... I happened across their community whilst doing / building something else. I wouldn't have known donating / subscribing were even an option if I didn't. I'm sure I'm not the only kind of person who subscribes purely based on the fact I get value from the software.
I'd like to believe there's a viable business model around these sort of things. And honestly, a much less ethical version of me says that there absolutely is when it comes to Data. I don't think it'll ever be mega profitable, but sustainable? Sure. The Omnivore team was like 2 devs and open source contributors. I believe you could get to a point where it'd be able to sustain that team.