That simple? No its not.

You need a way to move the lever with the jockey wheel. But it is not a simple lever...

You also need a way to take up chain slack as you go between different cogs. So that jockey wheel shown is actually just the upper jockey in a tension arm (cage) like you see on a rear derailleur. So you are moving the whole cage.

What I just described is basically a rear derailleur in terms of complexity. The only real difference is that the linkage moves the jockey wheel vertically instead of at an angle.

So in addition to whatever is needed to make the cogs telescope, you still have something hanging off the bike just as complicated as a current RD.

And regarding the number of cogs: the current limit on cogs is not just the chain line at the extremes. It is also how much space the cassette takes up on the hub. This telescoping cassette takes up just as much room.

And regarding the shifts: the chainline currently does not really effect the shifts much. Shifts initiate at the jockey wheel (where the chain first contacts the cogs) On the current RD system it is already lined up perfectly straight with the cogs.