I think mangadex can benefit from this simple change.
How? The load on servers will decrease. Some might say, maybe the load will decrease but what if number of visitors increase, it will balance out again. Well, I don't have the access to such statistics so only mods/admins can figure out that math.
Mangadex is a complex website to any new user and not welcoming (ease of use/eye pleasing/whatever you wanna call it) in any way.
So, if a new user comes here, and they at least can read a chapter without load problems, at a little compromise of quality, the majority can accept that. Maybe they later on, try to understand the website, but first they gotta stay. And if they want higher quality, they'd be able to do that as the time passes.
Now, the vice versa situation, if people face problem with reading(because of high quality pictures), it decreases the chance of them staying, which further decreases the chance of them trying to learn how the site works, which further decreases them ever trying the data saver option. So the point is getting attention first, everything else comes later.
It's not really a moral/ethics issue, in my eyes, you know like, we wanna provide best quality or something like that. It's all a numbers game. And the quality we're talking about here is not even noticable by the majority, it's not like data saver provides horrible unacceptable images.
That's my feedback on this subject.