Slowing down =/= Adding more notes. It seems that you are new to rhythm games, so please be aware that the number of notes is always the same, it will not be changed no matter how fast or slow it is. It is also included in many rhythm games for the players to adjust until they are satisfied, so I don't see a reason of removing it.
In fact, changing the scroll speed is made for people who want to be more comfortable at reading notes, but not reducing the difficulty. As more notes were added, it would be harder to read the chart for players. Hence the player would need the slider for changing the scroll speed. In the nutshell, if you find slowing down the speed would make it hard to read, then increase the speed until you are satisfied.
If you want to play an easier chart, change the difficulty, but not the scroll speed. I hope this helps.