Over time I've encountered many PNGs, GIFs or the occasional APNG that are poorly compressed, if at all. Not utilising transparency for redundant areas in animations, or people naively setting "Compression" to 0 when exporting PNGs.
This was the latest example I stumbled across just now post #5019102, I can shave 69% (Nice) off the filesize and get it down to 13 MB, just with a standard lossless PNG optimizer. This would be an ideal case for the post replacements beta, but this is an archival site so I imagine not having the same file as the source could be iffy, even if the pixels are identical.

I'd like to know if halving load times and bandwidth usage is a fair trade-off for replacing the source file, and if so, then I'd like access to the post replacement beta to assist with it. Hopefully this is the right place to ask...

We will almost always not accept visually identical replacements, we don't want optimizations unless the file does not work or is too large to be uploaded here
If we wanted optimized files we would optimize them on upload, we want as close to what the artist created as we can get

No.

This will create precedence whereby the post OP will get punished for using the original source file while some random dude "optimised" the upload and replaced it with theirs.
Moreover, they can do this for essentially every post with inefficient compression.

picture, in your mind's eye, what would happen if we started accepting third-party filesize optimizations as superior versions.
a small handful of people would be competing amongst themselves in a Pingo arms race, replacing the entirety of type:png, and all for no gain in visual quality.

Granted, I actually looked at some of the oldest large PNGs and it was vanishingly rare, the later you look. Basically, the default settings and compressors became nearly optimized for filesize, anyways.
So even if you did this, it's mostly a waste of time for all but the 1 in 10000 files that you gain more than say, 10% of compression. As far as optimizing alpha channels and stripping large non-PNG data go, that's a different issue. I've seen GIFs on sites that had a ZIP on the end. Might not want to delete those if they include the game files on cover of CGI set. ;) Yes, some people did this with ancient PC88/98 h-games. I think I've seen original authors do it as well for really small games. Now as far as malware goes, there's a reason that sites often strip that stuff, and why almost no one allows SWF uploads, anymore.

Thanks for the answers, I'll stick to hunting down the best source files I can.