Things are always resolved one at a time, but that doesn't mean they aren't representing simultaneous occurrences. If you step into a zone with multiple Enter Zone Effects, they all occur the moment you enter that zone even though you have to resolve them one at a time. When an attack is made against you, the Damage and Effects are occurring at the same time, even though you may have to resolve them one at a time (see Corrode). The same is true for Upkeep. They are all happening at the same time, you just have to resolve them one at a time to make sense out of it within the rules. However, these simultaneous events do not allow a chance to reveal enchantments despite them being resolved one at a time, unless they have substeps to them. That's a big reason for grouping them into phases and steps to begin with. A phase or step tells you "this stuff is all happening right now." They tell you what is happening simultaneously and what isn't by telling you what can be interrupted and what can't.
Now, if the different effects during upkeep count as steps, then that would change matters, but there is no place that they are ever referred to as steps. Up until this point, steps seemed to be a very clearly defined term with very specific meaning. Everything that was a step was clearly labeled as such. Now it seems like we can call anything we want to a step. We no longer have any clear rule for what can be interrupted by enchantments and what can't, because we get to arbitrarily decide what counts as a step. Does rolling the burn die and applying the burn damage count as two different steps? Does rolling each individual die during an attack count as different steps?
Before we had very clear rules for what could happen when, and now it is all subjective and open to argument. It would be a rules nightmare to try and clarify each and every possibility if we destroy the framework outlined in the rules and open "step" up to subjective interpretation.
