You are correct. Websites, the stack to supply video encoding, even scalability is a solved problem.
The hard work isn’t technical, it’s getting people onto your platform in the first place (marketing), getting people to continue using your platform (retention) and the perennial problems of SaaS evolving with other SaaS platforms (how many dev hours are you willing to eat trying to keep up with the Joneses?).
SaaS, and in this case, SaaS offering content, is a losing game. You will either lose your shirt, sell your business, or become entrenched in a position whose inertia is difficult to break. How much of any of those you are willing to take a firehose of is the question.
Not that I’m happy about this or anything, I think competition is good.
But I never got readarr to work properly, it seemed to have a workflow that was unintuitive to me, compared to Radarr and Sonarr.