There are quite a lot of BDs where people question the dialogue level mix. I don't recall any offhand (think The Dark Knight was one such popular title), but do recall that Planet Earth isn't a problem for me since I watch bits of it on and off (it's always in my tiny media rack beside the BDP).
I know what you mean though, with Attenborough's smooth/even/calm voice... I would just bump up the center level (if appropriate for the dialogue in this case), I mean that's what channel level controls are for.
I am never afraid to adjust the center level, even to an "unbalanced" extent if the film is extremely dialog-driven and I don't want to miss a single breath and I'm watching it for the first time.
What would be really wonderful is if they recorded only dialogue in the center channel. You'd think that would be a no-brainer, but if anything I notice on BDs that even more than ever is being put in the center channel. So many have virtually all audio in the center, and even the front L/R only have very occasional effects/music. So bumping up the center level on those doesn't help at all re dialogue clarity as dialogue won't stand out more, it's all just louder. As the quality of the audio has gone up, the quality of the mixing seems to have gone down. Just my general observation, there are many excellently-mixed BDs too, mostly the big action and sci-fi ones. OTOH there are some 7.1 DTS-HD MA tracks that for all practical purposes are (center) mono.
I found the mixing on typical BDs required me to drastically upgrade my center channel capability. There is so much going on there now that it seemed worthy of better attention. I never really had/have an issue with the center on DVDs. I have no explanation for it beyond the mixing. They used to put a lot of what would be "mono" non-dialog front sound in both the "stereo" channels, now they typically just jam it into the center with the dialog.