All of this sounds very understandable. I wouldn't be surprised if that low 15% "aftersales" number is because the second hand market gets flooded with cheap half assembled or NoS models from projects that were just an impulse buy in the first week and the original buyers realize it just added to their pile of shame...
My views on the main topic:
AoS is a game in full development. Extra rules layers are still being added and invented. Endless spells and terrain are just the latest arrivals in this. Subfaction rules (stormhosts, clans, grand courts etc) are also a reasonably new layer. Spell lores are an old layer (FEC V1 still didn't have that for instance). So yeah, because of these changes to game depth, newer releases will feel quite a bit stronger compared to old ones who don't have them. This game almost needs a flurry of V2 books + expansions at this stage, and I believe that's going to be exactly what we are in for (or V1 for major factions that don't have a book yet, like ogors).
However, If somebody says: the latest models have the strongest rules, they are full of sh*t. Some of the more recent books also turned out to be duds in terms of gaming power (Nighthaunt being the most obvious example), so I don't think there is a clear 100% powercreep indicator. But a V2 book is always going to be more powerful than a V1 book (or no book)... I don't think they necessarily invalidate "older books" that did really well in the past though.