No, they weren't really doing it alphabetically. Not in the sense of starting at the beginning of the alphabet and moving on. They'd zap a bunch of companies whose names began with the same letter of the alphabet, and then move to another letter.
There are plenty of delinquent filers left. Renee has provided a complete list of them here.
The 255 dormant shells were Pinks, not delinquent filers.
Yes, if they want to litigate, that takes more effort. And as you say, they don't get much bang for their investigative buck, as most are broke by the time judgments are lodged against them. But White did wax rhapsodic about suspensions, so why have they stopped?
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