This title seems very premature, and the article itself doesn't justify it well. While there are reasons to not use Heroku (pricing, the security debacle) and be annoyed with them, I don't see any signs of them losing their position in their sector of the market yet. They are still the go-to if you're in the "small-size project, don't want hassle, set up everything for me please" space.

They might lose that position in the future due to many missteps, but at this point it's like asking "why did Microsoft fail?". It didn't. It might not be what it was, but it's still a pretty successful concern, and same goes for Heroku.

Having alternatives is very cool. I did migrate an app to Render, motivated by Heroku's recent security woes. But in the process of doing so, I realized that Heroku has accumulated a lot of plugins, conveniences etc. over the years. If Heroku dropped their prices to be a bit less egregious, I probably wouldn't bother moving.

It will take quite a while before anybody can match them on convenience, and eg. Render is definitely not there yet - it has the basics down and done right, and if the basics are enough for you, it'll work. But a lot of things you get out of the box one-click support for in Heroku will be a manual setup on the alternative PaaSes, with a community wiki article to guide you at best.