In the end running the old version probably doesn't cost Reddit much, nobody expects them to update or maintain it and so they have no reason to shut it down as long as it keeps working. It isn't worth the inevitable backlash.
Which is why they don't phase https://old.reddit.com out.
They know the contributors use the "old" reddit.
Also the reason why they don't publish those stats. Too ashamed.
I honestly think one of the biggest issues with axing old reddit is that old reddit users are disproportionately moderators (even though new reddit has unique mod tools). Reddit survives off of unpaid labour, it may be a bit wise to rock the apple cart too much.
For the vast majority reddit is just the 15 or so default subs ran by a tiny number of power mods regurgitating content. Has been for years now.