Well, even in hypotheticals, I'm not sure how many cases are likely to go beyond 3 layers. And that's just for journalism.
For practical functional applications, 3 layers seems high, and potentially a sign of over-engineering.
To get a little more technical about our hypothetical:
- I've used Mapbox in the past, and they included (at the time) a succint dual-attribution overlay for themselves and OSM as part of their APIs. It was well-designed and baked in (nothing for me to do).
- If I were to then take my own application and offer it as a service for others to embed:
1. There's a strong likelihood I'm offering a commercial service so attribution may not be required for my layer (provided service-charges covering white-labelling)
2. If I am requiring attribution, it might make sense for me to invest in direct OSM usage & cut out the Mapbox middleman.
3. If not, it certainly seems like my added layer would be the absolute extreme (and baking in a well-made 3-component attribution would be challenging but not insurmountable).