Well I would say the combo release are one way to get folks like you to upgrade at some point. You will buy it and have both, so you can give it a try. Not upgrading is like saying "color? black in white is good enough". The fact is that film (35mm for example) has a higher resolution than BluRay allows, technically, so BluRay is just getting you as close as possible to film projection. Saying "DVD is enough" just makes no sense on any kind of technical/factual argument, but it is anunderstandable economic argument. Wherever there is decent quality source material available, for the love of these movies, I wish they will arrive on BluRay, just like I want "Taxi Driver" on UHD-BluRay as soon as it is out, because if they did a 4k-Remastering then well... why not see it in 4K at some point in the future.
This is a hugely off-topic debate though. But this is the one reason I could imagine they package DVD with BluRays: as bait