Yes, you can maintain your fork for perpetuity if you can't/will not get your changes upstream. Why is that a problem?
If you're using any complicated FOSS professionally and you have SLA with your customers to say fix issues within day or two you don't have a choice anyway.
Because it's a ton of unnecessary work. And because of the other reasons I said.
> If you're using any complicated FOSS professionally and you have SLA with your customers to say fix issues within day or two you don't have a choice anyway.
This is true. I always try to upstream patches anyway though.
How do you define unnecessary work if this is... necessary for you?
You are already benefiting from getting the tool/library/system for free, so you can still compare writing the thing you need (necessary?) from scratch or adapting the FOSS solution — maintenance comes with both options.
When you invest enough and are lucky, someone else might just fix the thing for you or pick it up and maintain it for you — but do not count on it, and you are good.
If you're using any complicated FOSS professionally and you have SLA with your customers to say fix issues within day or two you don't have a choice anyway.