I think a lot of people are under the misconception that getting good at streaming in osu! requires you to train your "finger muscles", and that stronger muscles = faster streaming. That's just incorrect. All the muscles you need for osu! are to press key switches down. The activation force of the keyboard may sometimes feel high when you are in a middle of a deathstream, but I certainly hope that you don't need any "training" to press a single key down. Practicing for osu! is different from other activities, such as sports, and training with the goal to just strengthen muscles on you fingers can lead to consequences that may even cause finger injury.
The most similar activity I can think of that compares to osu! streaming is playing the piano. Playing the piano at a high level requires speed, accuracy, and stamina, just like osu!. Likewise, training to play the piano is different than other activities. Professional pianists don't exercise their fingers as professional athletes do with their bodies.
I think almost every pianist knows the story of Robert Schumann. He was a famous classical piano player and composer. Wanting to improve quicker he designed a contraption that made it so that pressing the keys on the piano required more force, hoping that he could strengthen his fingers while also allowing them to be faster. It was like the an upside down bench press for his fingers. Schumann ended up injuring his hands and was forced to end his piano career prematurely .
Now then, how do normal pianists train? They repeat hand and finger movements. And they repeat them A LOT. Professional pianists spend hours each day to practice these movements, all to develop a muscle memory that allows them to play quickly and to allow them to have more stamina. The more you play, the better you get (hey, that sounds familiar).
Finally, how does all this relate with osu? Simple. Playing long stream maps just aren't effective at training speed and stamina. Streaming is mostly muscle memory, which is tied with the brain, not your muscles. I'm not saying that those maps are useless though, as they can still help your finger control, but there are better ways to practice streaming. The way I see it, stream practice maps are only used for more for confirming your streaming capability, rather than improving it. Playing fast, awkward, burst-heavy maps around with bpm a bit higher than what you are comfortable with are the key to getting better streaming. Playing those maps builds muscle memory that will be immensely more helpful than any long stream map, and of course, playing more helps as well.
But that's just my opinion. Oh and sorry for the long post.