*Know I'm a bit late in replying to this, but anyway....*
Actually there in lies the fallacy in your comparison, comparing Imfernape, a Pokemon which Ash caught in that particular region, to Pikachu, who is Ash's lifelong partner, has been with Ash since the start of the show, has an already long established unbreakable bond with Ash. In Infernape's or say in Greninja's case, the writers had to show Ash working and training hard with them, going through many hardships and difficulties, in order to unlock their special powers they inherently possessed, a.k.a., Blaze and Ash-Greninja form respectively. Infernape and Greninja were Pokemon that Ash caught in that particular region, they didn't have an already established bond with Ash, so the writers had to show their development via development arcs to establish their strong bond with Ash which would justify their special power, a.k.a., Blaze and Ash-Greninja form respectively.
But that's not in the case of Ash and Pikachu, they already have long established, strong and unbreakable bond, they are life long partners. So why would the writers need to Ash and Pikachu training hard or going through a tough development arc to obtain Pikashunium-Z? There's hardly any need for that.
So no, you can really call Pikashunium-Z as a cheap power-up considering Ash and Pikachu long established unbreakable bond.
The same answer goes to @Locormus