Honestly, I hope I'm wrong as well! Getting regular updates to armies would be a good thing, which is why I'm hopeful the app is going to lead to that sort of change.
The thing is, old models aren't an internal driver. They're a sunk investment. The internal driver (and projects that get approved) is the new models, so that's where the focus is.It's that easy. GW does not have to make new models, in order to sell existing models.
Is it stupid? Yes. Does it mean their operating model should change? Also yes: there should be more focus on existing product lines and getting the most possible value out of them. But it's really going against what we know of how GW operates, and a ship/corporate culture that large takes time to change. I think they're shifting though.
This all ties back to why I don't think 40k is a good place to look if you want a good game for tournaments: it's really not set up to support it. The rules are written to provide a reasonably satisfying experience as a way of using the models players have invested in, but they aren't going to win any awards for game design!
Edit: also, as far as I can tell, GW doesn't actually keep all that much stock: if something starts selling loads, they're having to make more of it. Old stock that doesn't sell isn't actually that much of a cost to them.