Counting is not just to assess the value, it is also for the purposes of Army draft, taxes (Mas), including sending workers as a tax. Megilla starts and ends with taxes, so that got to be important.
What would the be modern equivalent? not just a census, but maybe an election? Don’t do elections for no reason just to re-assess your power?
It is to David’s credit that he moved to Teshuva. He could have said that extra 100 people a day is not that much. They might have died from a heart attack anyway.
Interesting,ly Israel had highest number of niftarim during this whole pandemic – 101! – on Jan 20 (blame Biden?), the only day over 100. Population now and then look similar. David counted 1.3 mln fighters, so 6-7 mln people