Why would Blizzard care about retail subs when half the playerbase is on Classic and these print the same money with the same sub without having to invent any new content whatsoever because the path is already laid in front of them?
Add new "inventions" like TBC boosts and "character cloning" and deluxe editions with new mounts and you're squeezing more money without having to increase your sub count.
There's very little financial motivation for Blizzard to keep investing in retail if it's classic what keeps the franchise afloat. If only half the subs are on retail they can half-ass it and just drop another 6-month sub with a free mount if you give them the credit card numbers (tokens / prepaid time don't work).
Classic launched halfway BFA, we would have to compare current numbers to pre-Classic launch numbers to have any form of comparison.