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Modern ABRs are actually quite sophisticated, and in most cases you're unlikely to notice the forward buffer limit. Unstable connection scenarios are going to be the exception where it breaks down.
For best user experience it's of course good practice to offer media offlining alongside on demand, but some platforms consider it a money-making opportunity to gate this behind a subscription fee.
My internet is intermittently like 100mbps and 256kbps. It sees the 100mbps and acts like it's going to be that way forever, so doesn't buffer the whole video while it has the fast speed, then drops entirely when it slows down.
An ABR is generally going to make an estimate based on observed bandwidth and select an appropriate bitrate for that. It's not out of the question that you run out of forward buffer when your bandwidth takes a nosedive, because the high bitrate video is heavy as all hell and the ABR needs to have observed the drop in bandwidth before it reconsiders and selects a lower bitrate track.
I'm not familiar with ABRs affecting the size of the forward buffer, most commonly these are tweaked based on the type of use-case and scaled in seconds of media.
If that were true then users wouldn't hate and complain about it. This post existing is proof that it's shit because clearly it's not as seamless as you're making it out to be.
And then there is youtube which just discards the whole buffer content each time an ad plays. Very sophisticated. Although knowing google that behavoir is likely on purpose
I've been running adblockers since the beginning of time so I can't speak to this behavior. Spontaneously it seems a bit amateurish, though.