Just because someone has a successful console, that does not gurantee a successful handheld. I already mentioned the PocketStation as proof to this, and I'll mention the Wonder Swan too. Square did develop for both, so even with big-name developers on board to make games for a new handheld, it's not going to mean success in the short-term or long run.
By the way, you're the one comparing apples and oranges. You're comparing today's console market leader with yesterday's failed handheld upstart. If you want to make the right comparision, realize that Nintendo had something like 9 or 10 million Game Boy units installed when Sega came to market with the Game Gear. When that didn't work, Sega tried to marry its Genesis (which at the time, was the market leader, just like Sony) with a handheld in the form of the Nomad, and that still didn't work. If you add up Game Gear and Nomad sales, it's still just a fraction of total Game Boy original sales.
Even though the system was better, the fact of the matter is Sega was trying to infiltrate a market that already had a proven leader in Nintendo's Game Boy, and it ultimately failed simply because people went with what they had been going with for the three years or so before it. You're banking on the fact that PS2/3 users will automatically buy a handheld that can link with a supposed Sony handheld. Big deal, Nintendo will have been doing that for the year before. There are about 15 million people in the world with a GBA and no GameCube. Granted, most GameCube owners own a GBA, but the point is that people buy a handheld first for its own merits, then for all the bells and whistles after that. That's why every single handheld that is not a Game Boy has ultimately failed. And that's an almost 15 year history.
What's going to happen in five years? Who knows? That's too far away to even begin speculating on what might happen. Personally, I see no viable way that a handheld will go to a disc format, because a disc can only be so small. Sony or Microsoft might have some sort of stake in the handheld market.
If you think Nintendo is just going to sit back and let people eat into its handheld position, though, you're crazy. Nintendo has proven before that just because you're number one doesn't mean you sit back and let the others catch up to you. If they would have thought that at all since 1989, we'd probably be playing games exclusively on our cell phones by now.