No. That fee provides access to characters up to the max level permitted by the expansion owned (110=base, 120=BfA). Not paying the fee still allows access to the account and Blizzard’s servers, up to 50 characters, but prevents any character over level 20 from being played.
I think it is a good example of how they plan to make things work because of the way it was set up.
I paid for the Virtual Ticket via the Battle.Net account, not picking a singular WOW account. If the Demo was just a temporary access account, it would have only needed to be a single Demo account associated with the Battle.Net. (Which actually is how the PTR shows up - just a single special account.)
Instead, as I say above, I have 3 WOW Retail accounts, and Blizzard automatically created 3 WOW Classic Demo accounts using the same names. (Two of my accounts have names I made up, because they predate Battle.Net, so it was really obvious.) Why make three, when the Demo was associated with the Battle.Net account, not a WOW account?
I suspect it’s just because a trial account is just a modified full account. Perhaps the character limit is more deeply coded than we know, and it would be more tedious to adjust it than to leave it as is.
Honestly the thing that would make the most sense to me is that they either compress inactive account data to save space, or the account data isn’t really “that” big of a deal anyway. At a peak of 12 million accounts, but a total created accounts in the 100 million plus range, if the storage space was really that bad we’d be seeing some issues from that.
My guess? It’s just not a big enough deal to do anything about. Particularly given the fact that Vanilla will have basically half of the storage space they do now. Most players will end up with 16 slot bags for the most part, compared to our 30 slot bags. No reagent bank, no transmog, collections, etc.
Character storage space will be pretty lean in Classic vs Modern.
Someone out there somewhere is screaming because they can’t have 50 characters on one realm with faction overlap with a million character slots total for classic.