You guys are turning this into a technical debate when it's not. If it were about technical details, then alternating is superior, since you can just train streams to sort out your speed/stamina.
Fact is, it's not that simple. You can argue this forever (as it has been going on forever already), because like I said, everyone has different walls to overcome and some may not apply to certain people who have success with alternating. Generally people have an easier time grasping the game and learning to be accurate when single-tapping.
If all you are going to say is, "oh, but that can be sorted out with practice" - then you don't have an argument. We aren't comparing the skill cap, we're comparing rate of progression, since it's quite obvious that no one is there yet and alternating has a lot of potential. Many people share the opinion that alternating takes more effort/practice since there are extra variables... like practicing to do patterns in both directions with your fingers, and to do that fast and accurately.
Anyway, that's not all there is to it. If it were just a speed/stamina issue, how do you explain why alternating players don't challenge themselves? Who would get comfortable so easily? There has to be another reason and I think reading has something to do with it because alternating players aren't anywhere near challenging their speed/stamina on typical maps. You actually have to go to deathstream maps to practice something you rarely use.