Some people get quite upset about the use of ‘decimate’ to mean a large reduction in a population (see sense 3 below). This is usually because they think its usage should be confined to the so-called original sense (see 1 and 2 below). By the way, they’re generally the same people who’ll also go on at you about splitting infinitives. Ignore them and proceed with your life, smiling broadly. Perhaps shout a breezy, “Love you!” as you swish by.
I say the so-called original sense because there’s apparently some evidence that ‘decimate’ had yet another, possibly earlier, meaning: to tithe (ten percent of one’s earnings). If you’re interested in this, read more about it here from the Oxford Words blog.
Either way, I think it’s absolutely fine to use ‘decimate’ as a way of describing a substantial loss of something, be it a population or budget or whatever. But before you do, it’s good to stop and think. Did the ‘decimation’ wipe out everything, or nearly everything? In that case it might be better to use ‘devastate’.
The moral of the story? Pick your words carefully. It’s perfectly fine to use ‘decimate’, as long as you’ve first considered whether ‘devastate’ would be more appropriate.