The PSP is actually performing fairly. The game drought looks like it might see an end soon.
The gameboy, GBA, and the DS are such icons of gaming, that yes, the PSP certainly does take a backseat to them in the "rememberance" category. The "legacy" factor of the PSP isn't necessarily a strong one.
That said, the PSP isn't dying, but it also isn't thriving. Let's not get too optimistic about its future. Have you seen the upcoming DS lineup? Square-Enix is to the PSP as Blizzard is the the PC. Square has about double the amount of upcoming games hitting the DS next year...however the DS library could take or leave them and it would still be amazing.
If it wasn't for the DS, the PSP's current level of success would look and feel monumental. It really did do well this gen, to be honest. However, with the DS there, the comparisons are inevitable. When your competition is one of the best consoles of all time, and one of the highest selling...you aren't gonna look so hot no matter how well you do.
This is all my opinion, of course.
I don't need your console war.
It feeds the rich while it buries the poor.
You're power hungry, spinnin' stories, and bein' graphics whores.
I don't need your console war.
NO NO, NO NO NO.