Cost efficiency refers what it costs the cruise line to run the ship per passenger mile. It does not refer to the cruise fares for the ship.
I think your view of cruising is a valid one, but I don't agree it is the only one. I also know, having been on Oasis, that it is not as you seem to think; nothing but an exploded version of a smaller ship. It is a completely new design in ships with great traffic flow and plenty of space per passenger. You will feel more crowded on many other ships fron Carnival, NCL, Princess or even older RCL ships than you will on Oasis.
The best analogy I can think of is this: Would you tell an amusement park enthusiast to avoid Disney World because it is too big? No. The size is what makes it an attraction, it is what makes it possible to provide a diversity and bigger and better experiences, because of the ability to provide more because more people are paying to see it.
I am a fan of big things, because I am a technology nerd. Some people want life to be simple - so they live in cabins in the woods. As a kid my favorite cartoon was the Jetsons. I don't think things are bad just because they are new and big - I like that.
There are still plenty of cruise ships out there doing the kind of cruising you like. Oasis may not be your thing (although I would bet you would be surprised how much you liked it if you tried it) but it IS a cruise ship, with plenty of sea views, that will visit islands that do not feel overly crowded. It is scheduled to go to Jamaican (Falmouth) which no other cruise visits visits as soon as the dock is ready. It also stops in Labadee on each cruise, and will be there alone when it does.
If RCL had not built Oasis, they could have built two Freedom-class vessels for the same amount of money, and your ports of call would have been even more crowded.