Plus, they WOULD make alot of money...
Like you said they would have to optimize old games for the new generation...that takes time and money. They could possibly sell just as many to those who haven't pick up a Cube (there are lots), while not hassling themselves with the things needed to make a new game.
Also, I for one haven't bought all the good games for the cube, and I'm thinking that I won't till at least the launch of the REV. Having a PGC to play around with would make for an even greater incentive.
Plus, the time not spent making new 'old' games, could be better spent making completely new games that I would be very interested in.
As for the Multiplayer aspect, I'm hoping at least that the solution could be based on the Wavebird. Make each hand-held a 'wavebird' in essence and you've solved HALF of the problem. The other problem would be to somehow have each screen used.
I bet people would still buy the old multiplayer games due to the fact that the REV can play them. I don't expect this to be launched until after the REV (I did at one point think it could be launched next year, but that's not a good idea). And if the REV sells through, having the next gameboy with GC compatibility would be a marvelous link between consoles.
People will already have the ability to play the same games on the REV with no additional costs. That alone can make up for no old multiplayer support. In addition, Nintendo is just as known for it's Single-Player games as it is for it's Multi-player games (even more so this generation). So I doubt there would be any major complaints.
It works anyway you slice it (though multiplayer is a bit off). I've said it before and I'll say it again, the ONLY problems is the cosmetics and overall effecientcy of the product.