A solution could be to make the usernames available after a random period of time (for up to a year), so popular usernames aren't released all at once to be grabbed by a squatter. With a staggered release this at least allows the chance for someone legitimate to stumble upon the username before a squatter arrives.
If squatting Twitter names becomes even marginally profitable you can be sure that squatters will be setting up bots that will monitor the availability of deactivated usernames that were popular in the past and that as soon as these are released they will be taken. A delay of the kind you suggested will not stop the bot writers.
On the other hand, my username (arp242) is already "squatted" by an account that never tweeted or had any other apparently activity. I don't use Twitter much, but people have erroneously "mentioned" this account on a few occasions.
I doubt they'd free up the usernames. Username reuse has problems with reputation hijacking, either for paid likes, spam, or making it look like the user said something they didn't. It also creates a bad experience with historic @users in tweets, and no one wants that bad PR tweets pretending to be from someone deceased.
I imagine they want to free up accounts that never did anything. You should be able to safely make a handle available after deleting an account if it has never tweeted, has not been tweeted at, and hasn’t logged in in a long time.
