I always assumed Spock was the ultimate in an un-recastable character. Now I'd be willing to see any recasting. Go ahead and recast Han Solo for all I care!
But the reason TNG wouldn't be treated with the same respect as TOS (and Abrams did treat the characters with a fair amount of respect) is that the TOS characters are truly iconic in a way the TNG characters aren't, except for Data.
An iconic character transcends any given actor, so the role can be handed off from one to another. That's what being iconic means. Data is like that. Picard isn't - he's specific to Patrick Stewart. So a new actor playing a character named Picard would be inevitably different. But I wouldn't mind seeing that new character named Picard. He might be cool.
Not to mention that the whole thing would screw up already established Trek history, just like that recent movie.
Nope, it's all in a different reality. The original reality remains untouched. We aren't seeing those characters at all. So in that sense, nobody is actually being recast. A new Picard might be strikingly different because he's literally a different person, born at a different time, born to the same father but a different mother, born to a different father who was the brother or cousin of the original dad or even some other more distant relative named Picard. Once you futz with the timeline, anything's possible.
If Star Trek can't come up with new scenarios, new stories and new characters, than they could just call it a day and cancel the whole thing, leaving TNG, DS9, Voyager and the other great series to fanfic and our own imagination.
The reason TNG would be revived is not lack of imagination on the writer's/producer's part but lack of spine on the network's part. Networks like to reboot existing properties because it gives them the illusion of predictability and safety, even when the reboots rarely have anything to do with the originals. TV is run by cowards.
And if you want to hold off on Star Trek
until TV is no longer run by cowards, you'll be waiting a very long time. Star Trek
needs to get back on TV the way all the other worthwhile TV shows do - in spite of the suits. That's how Ron Moore got BSG
through the gauntlet. He threw out a known brand name that gave the illusion of predictable safety and then proceeded to ignore that brand name and make something good instead.