What's on my mind as a main source of this inertia is sequels of popular series. You see more games going multiplatform, but the important thing is it's hardly going away. As long as you choose this platform you can play a sequel of your favorite game. Clever customers may seek a cheaper option in other platforms, but not all people are like that. Then again, the cheapest options can be found in the portable segment so it won't be an absolute basis of decision for consumers in the high-end console market.
I don't think it's just "clever" customers who will seek other options but everyone who sees the same games lineup and a significantly cheaper alternative. If their favorite title doesnt appear on the cheaper alternative then that's one thing but if the cost difference is large enough it may cause them to consider other exclusives that have historically been foreign to them if they were a "one console" owner.
Regarding portables being cheaper options - to some they are not options. I have never considered a portable as to me it would be a waste of money. If I have time to game, I'm at my house and I might as well do so on my tv rather than a 2-4" screen without surround sound and less comfortable grip.