Even as a peripheral I'm not sure there's a big market for this, Wii U demonstrated that off TV play isn't something that the mass market find terribly exciting.
I mean the Wii U was $350 with very few games, with absolute aweful marketing, so I'm not sure it should be used for comparison. As a peripheral, if this could sell 8m that'd probably be a success. Worth mentioning that unlike a platform (PSVR/Wii U) this doesn't need dedicated software support. It's no different from the costly £203 dual sense controller which is bound to only shift a few million units over its life. Even the steamdeck has been rumoured to only be targeting no more than 3m for it's first 12months on market. So expectations should really be kept in check for what sony thinks the potential market for this is.
It'll all come down to marketing and cost. There's a decent 10m audience there. Playing a PS5 from your phone is not appealing to anyone, a dedicated PS5 tablet however...