>do you really think regulators should serialize investigations and actions, only moving on to the #2 worst offender once the #1 is sorted?
Actually, yes. Because if they take down the #2 worst offender, that leaves #1 as the default option for consumers, and strengthens #1's position at #2's expense. It makes the problem even worse than before.
As a consumer who doesn't want to use Apple devices, and uses Google because that's basically the only other option for many things, anything that greatly harms Google harms me. Take down Apple and create more competition, and then I'll have more options and I'll be able to maybe choose some things other than Google. After Apple is broken apart, then Google becomes the #1 problem, so go after them at that time. (Yes, I know this particular case probably won't have that much effect on Google.)