[popo]

And you can ride the horse named "equivalence" until it's completely dead

It's already dead. I think I ate it the last time I had a burger.

Do you think it will be possible one day in future to get an APS-C camera completely equivalent to a FF or even to Medium format DSLR?

Within some limitations, we may already be at a day when a mirrorless APS-C system can be near enough full frame equivalent. Just add one of these. Ok, the limitation is currently that you have to use EF lenses on a NEX unless you like manual options. But in principle other full frame lenses could be used on more APS-C sensor mirrorless systems. With this reducer it is (near enough) equivalent to the native full frame system. You may start arguing about quality now.