I think there's already a bit of shifting towards this in the console-world. I believe you can buy a keyboard and mouse for PS2, so you can chat with other people online, the mouse obviously mostly intented for shooting games.
But having an all-in-one machine will prolly cost a fortune as mole says. And i don't think that sony, nintendo and other x-boxes would want to merge into one powerfull platform either.
But in theory, a console is what you propose, only that you have a big disadvantage of seperate titles for different platforms. Like Farcry for PS2 would be excellent
But since for example PS2 is out for some time now, it can't compete with the topgraphics from the newest games on pc, so you would have to go for the next generation console again, which means the same as upgrading, as mole also mentions.
Btw why would you want it connected to a monitor? It should have both abilities imo, i like playing on a big tv-screen. Only advantage of the monitor would be if the gf wants to watch tv, i can plug in my cable in the monitor and start playing there.
). So the knife cuts on both sides.