I have to believe whoever writes the tome has to have a reasonable idea of what is 'normal' now though?
I have lost count of the times I have been asked "Your lances only have 1" reach?", "He only has a 5+ or 4+?", "Any rend? ... It's a dinosaur?", "Those are one wound each?" The powercreep in even basic stats is one of the main reasons we need an update, to my mind.
Yeah, I imagine that unless something truly crazy happened and Seraphon started winning tourney after tourney, the
recent tournament listings wouldn't matter too much. They're design turn around for battletomes, updates, and expansions is usually at least a year (more commonly two years), so if we do see a Seraphon update, the details for it would have been developed many months ago.
I think it's
reasonable to expect an update for us some time in 2020, but I honestly couldn't say when or how good it would be. It
seems like the less favored factions get a passable to decent new battletome and
maybe an endless spell
or a scenery piece (spells for Gloomspite, nothing for Cities of Sigmar, nothing for Orruk Warclans, scenery for Ogors), while the favored factions get the whole nine yards and a strong battletome. Personally, we don't have any mechanics that I'm super attached to, so I don't think I'll feel like I get burned if they take something away (R.I.P. Great Companies; you were so much more interesting than bonus command points).
That being said, the whims of GW are inscrutable and cannot be known by the Euclidean-oriented mind as
we understand things.