Annual events in a MMO are just that, annual. You shouldn't expect to get every achievement on every character across all your accounts in one incidence of an annual event - it may take several. That doesn't concern me, and I have 28 characters !
I do not think Zos should limit their event and related achievements to make it so players can complete the achievement on all 15 characters in one cycle.
Considering we will likely get even more character slots available to us with time it seems even less practical.
During this holiday time I'm thankful that I'm not so insane as to feel compelled to get a pointless achievement on every one of my characters.
I've tried to keep up with that in the past, but ESO repeatedly pushes me away from that.
Last year you needed Morrowind to complete the PvP ones. Forced pass.
This year you could do the PvP ones without Summerset, but obviously the psijic passives would provide an edge to those who owned it. So another pass.
The 150 requirement for the Murkmire prologue dailies is bonkers even for one character. The notion of doing it on multiple characters is supremely unappealing.
And now there's this one with an account wide daily timer and trying to force players intro trials. Pass.