Back when Heroku first arrived, PaaS was a new idea. It was available ~2009, well before services like AWS Lambda existed.
This was such a paradigm shift that no one knew what to think of it. It takes time to build trust around such a big shift. The best way to do this is to offer to try it for free. Hence, the Free Tier.
People tried it, and were blown away by how easy it was to build services. It was so much easier than managing hosts directly. Fast forward to today. Engineers are generally comfortable with higher-level services. They know what to expect.
So the free tier is used as as part of the funnel to onboard new customers. Back in 2010, that funnel had a /lot/ of customers going in! Here in 2022, that funnel may be basically zero. On top of that, the free tier costs heroku to run.
Cutting off the free tier is in Heroku's best interest. It saves them money and allows them to focus on their current customers. But it does mean that there's no growth in the product any more, unless they offer something new.