I think Apple is much more deliberate than that. And I wouldn't characterise it as "shadowy motivations", it's just good business sense. The iMac forum here, as an example, is full of people buying iMacs with 8GB RAM and then adding their own from third parties. Why would Apple let people continue to get away with that, when they can force people to buy it from them and capture all of that revenue (and then some, given their markup)? It's just good business sense.
The corollary to your argument is that Apple's hardware engineers couldn't figure out how to "reuse the same case" and "redesign the internals" and "keep the RAM door", and were forced to give up and sacrifice the RAM door because it was too hard. I don't believe that for one second. This argument also requires you to believe that the same unfortunate compromise occurred for the other Mac models too, which have also progressively lost their access to RAM - rather a remarkable coincidence!

In my opinion, it's much more likely that this is evidence of a deliberate business strategy.