MW doesn't have a policy that their third definitions and below aren't as accurate as their first or second definitions (and shouldn't be used). I suspect you made that up.
Webster's Seventh Collegiate Dictionary, 1967, defines usury... explaining the order of definitions thus:
"In general, the order of definitions follows the practice of the Third New International, where the earliest ascertainable meaning is placed first and later meanings are arranged in the order shown to be most probable by dated citations and semantic development. This arrangement applies alike to all meanings whether standard, technical..." (etc.):
http://www.perfecteconomy.com/pg-what-is-usury.html Unless they've changed that policy, the original etymology is listed first in MW only because it's the oldest usage.
If you perceive your role as language police to be that of enforcing only pre-1900 word usage and original etymology, we're going to have to limit our speech considerably when we converse.*
*I was going to say when we "type" - but I see it's a stretch to use the word in that way. The more accurate meanings (right?) are:
1: to represent beforehand as a type : prefigure
2 a: to produce a copy of b: to represent in terms of typical characteristics : typify
My meaning is listed 3rd in MW, so I don't want to use that.
3: to produce (as a character or document) using a keyboard (as on a typewriter or computer); also : keyboard
darn, I just looked up converse, and that also is inaccurate word usage.

1archaic a: to have acquaintance or familiarity
b: to become occupied or engaged
2 a: to exchange thoughts and opinions in speech : talk
b: to carry on an exchange similar to a conversation (as with a computer)
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Bottom line - modern usage of decimate = wiping out a large chunk of something. Archaic usage = wiping out 10%. Totally decimate wouldn't mean every last bit is destroyed, I'd agree with that, but I think the implication is that the decimation is severe enough to render the thing ineffective for all practical purposes. Completely decimating a military force doesn't mean every last troop is dead or injured, but it would imply that the force, as a whole, is no longer a functioning cohesive unit with any effectiveness.