Netflix and Spotify wouldn't care. After not finding an app in Play Store a quick google will bring them to an alternate store or something like that, it's not Apple's walled garden.
It can only affect the apps that users don't know they need.
I think the value of Android's relative openness is often understated on HN, but in this case I don't think it helps. There is a very substantial chunk of users for whom "a quick Google bringing them to an alternate store or apk" is a hard blocker in terms of interest and/or ability.
It's hard to say if, for the average user, it's easier/more likely that they will get to the Netflix app (whether on official store or not) by clicking a link from the Netflix website than by doing a search for it in the Play Store. If they see a Netflix ad with netflix.com address on it I imagine they'll just type that in their mobile browser. A page would open, which detects the platform and prompts them to click "here" to install app. And now that link can be outside of the Play Store, the user doesn't even visit it.
But for all those that might mistakenly look for it in the Play Store, you can have a stub app there that just redirects them (opens a browser page) to where to get the real, outside the store app,
This wouldn't be the case had it occurred when Netflix or Spotify were getting started as opposed to being established brands. Its also not clear what people presented with the need to side load a secondary app store would just get frustrated and give up.
That's a losing proposition for end users. Nobody really wants "just google some alternate store and install that" to sound reasonable for anyone (with a few exceptions).
It can only affect the apps that users don't know they need.