Yes but in TNG when the Enterprise was doing those high warp speeds, ot was made clear that they could only do it for short duratins.
As you say they don't say how long they could sustain those speeds, but by using the word sustainable it would tend to indicate long periods of time.
As I said they could have removed the word sustainable without having a dentremental effect on the overall context.
That's what I said.
Does long mean hours or months.
I'm thinking of Tin Man were a Romulan ship blows up because they push their engines past specification trying to beat Enterprise to their prize... Although the script doesn't completely connect with my memories. I may be splicing it with the one where Kirk keeps going faster no matter how much Scottie kept saying "Gid lahrd MIn! ye guhnna Kull Is Arrrl!"