At least on metered connections, indeed. With HSDPA/3g this was not a problem, a few hundred KB/s is plenty to load text (including jquery and the like) and images just fine but you'd notice if it took oddly long. Then came LTE/4g and you can now download hundreds of megabytes in a few seconds when you've got good signal. When it came out, most people's data bundles were on the order of 500-1500 MB iirc. I guess nobody was a dick about it since I never heard of bundle exhaustion attacks, but what surprises me more is that few people mention running into the issue by accident
Now we have 5g and I'm still wondering what to do with speeds greater than what 3g+ (HSDPA) could offer, namely full HD streaming. On a rare occasion it's nice to download a file fast, but do I need this to be enabled for me all the time and run this risk? The lower latency of LTE and increased capacity of NR are nice but this constant race of more and more gigabits just to... do what, sell bigger data bundles? There are purposes for this (like wireless internet for homes in rural areas) but not mainstream mobile daily use. Range/coverage was the primary issue and still is to this day. The faster rates require good signal to function at all. Companies are turning off 2g and 3g, the things that allowed you to still get a few KB/s to send a message that you'll be back later, or get a weather forecast, when you're out hiking. The newer versions are just as good when you're in range and worse when you're out of range, so there is no point. I don't know how the incentives are so misaligned, I guess not enough people experienced 3g with good signal (nowadays when you see 3g, you're just out of range and that's why it feels like it's not working) to know that they don't need to upgrade to a plan+phone that can exhaust their data bundle in 90 seconds, or their monthly FUP allowance in a matter of minutes?
At any rate, yes I agree with your post emphatically, but it doesn't seem like a task specific for browsers to me. Any software can decide to pull an update or other data file (like an offline map file or large video), also while tethered so the downloading software might not even know that it's on a metered network, and cause the same problem