Heheh, when I asked this question a few days ago, I made a rough guess of the BP in active accounts: 200k. Not bad!
Interesting to note it's about half the total supply of BLURT. I'm curious to see what other numbers you can dig up.
"...how much truly belongs to active accounts?"
The money in inactive accounts also "truly belongs" to the owner. Even if the only known holder of the keys dies, or loses the keys, the money doesn't become the property of anyone else. In some cases, that means tokens will sit motionless for months, years, or effectively forever. Nothing we can do about that.
Whether it's someone who missed out on the creation of the Blurt blockchain, has passed away, who isn't paying attention to Blurt, or even Steem's ghost accounts, basically these are someone's wallet and will remain so ad vitam aeternam, so touching these wallets to burn their contents is like touching someone's account and burning their BLURT.
Regarding exchange accounts, I consider them as an extension of the exchange on the BLURT blockchain rather than someone's account. The client accounts of the exchange are centrally located on its own servers. Burning BLURT from the airdrop on exchange accounts, however, doesn't dismiss the question of how an exchange like Probit, for example, which is our only legitimized through its listing on CoinMarketCap and Coingecko gateway to the world of other cryptocurrencies, might perceive this gesture and how it could impact its trust in the security of its BLURT on the Blurt blockchain. Additionally, any exchange potentially wanting to list BLURT on its platform might be deterred. Such a complex choice, if executed, must be done in a very professional way, be very rigorous in the selection of accounts, and meet certain strict criteria in its implementation, such as a multi-channel advance pedagogical and highly informative notice schedule before its implementation.
On the other hand, there is the question of the security of the Blurt blockchain itself in the event that an exchange like Bittrex which is closing down sees one of its employees leave with the keys.
Of course, it would be preferable for the Exchanges themselves to send their BLURT to @null, but unfortunately, we don't live in an ideal world hahaha.
I wonder how opposed everyone would be to something like a dead-man switch, wherein after x time (a year? 10 years?) an account that has been completely inactive for that entire time will "volunteer" to have their Blurt burned. This is probably a moot point anyway since nobody with those stale wallets agreed to that to begin with, but I am curious to hear you take on it.