Yep. That's the argument I've been making for a couple weeks now. Mainly, that Apple's actions are anti-competitive, even if they may not match the current definition of monopoly. If the harm is great enough though, then we should maybe think about extending the definition of monopoly or enacting some more specific laws restricting anti-competitive actions that match the modern market more accurately. It's not like the current laws from the the constitution, they were added in the late 1800's and early 1900's.
At a minimum, some consumer laws that protect us from people locking down hardware to specific software in some instances (and thus allowing competition on Apple hardware) may help in some small ways overall, even if it doesn't directly address the App Store and OS coupling. Being able to (legally) run Android (or something else) on an Apple phone would change quite a lot, IMO, even if it took a while to do so.