But that's not what most logging systems that log to binary files offer. They give you specs (example: http://www.freedesktop.org/wiki/Software/systemd/journal-fil...) and tools.
Binary doesn't have to mean closed/opaque.
A binary file from whatever logging system, OTOH, is effectively proprietary. Even if the logging system provides you with tools to work on them, you have to 1) know that it's a log file for that logging system, and 2) be familiar enough with the tools in order to work with it.
And the specs will be gone in 40 years. While ASCII will stick around.
If a binary format has an open specification, it's as future proof as ASCII. ASCII's durability is due to a clear and open specification that's easily implemented. Not some magic sauce that makes it instantly human readable.
That text you see? It's not what's actually in the file. That's just 1's and 0's like every other format. There's literally no difference between ASCII and any other "binary" format.
Longevity of log files hardly seems like a reason to pick an otherwise inferior format.
For example, many nuclear power plant in the west were built 40 years ago. Amongst the myriad of sensors, devices in a power plant, I think that most of them are outputting ASCII logs. There are still readable today. (Same can be said about avionics, space probes, etc.)
Now imagine yourself 40 years from now on, trying to fix or reverse engineer a very legacy system, you will have to recompile a journalctl from 40 years ago before being able to read anything.
40 years from now, you will probably be able to invoke journalctl on the system and parse the dumped output as plain text. Or call gunzip on the compressed logs, $DEITY knows if we will be still using gzip by then. And if the system does not boot, you won't be able to connect the peripherals anywhere else... :)
There's no tool out there that generates log files it cant itself read. So there's not going to be any "oh gee I have these files being generated and nothing can read them" situation.
However, there is just about near-zero system out there that generates text logs that it can itself read. Text logs are write-only for most logging systems, while all binary logs I know of are read+write.
Stepping back though this entire argument is absurd. Thinking about "whatever will those people do 40 years from now with the tools of today" is fairly braindead once you understand that the quality of the tools will affect their longevity. So if the logging system becomes an actual, factual problem over time, the tools will die off by naturally-artificial selection.