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.
> Why is that a problem?
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.