Why wouldn't there be, people would get tired of the 5 at some point
I know we are a while out on the initial reply but
Sony goes pop. Would not be the first megacorp to do so, though lower on the list than some others (they have money, fairly diverse income, credit rating not bad, lot of assets, own both tech and the means to make it... it would take some massive fraud to knock them out).
Sony gets a new CEO, decides the playstation division is not worth the effort and kicks it in the head.
Sony gets a new CEO, decides the playstation hardware division is not worth the effort and goes software only.
Sony gets a new CEO, sells the hardware division to someone else and they take over. Might technically then be a PS6.
Consoles lose relevance. Someone might crack streaming games, phones might take over in a big way (give tablets happily handle emulation of devices up to the PS3 at this point that is a fair bit of native power, or indeed everything the Switch does and the Switch was a middling-good phone when it was released never mind now).
We take a fork and go into the good universe where console makers are like DVD player makers and everybody builds to a standard. Technically a PS6 might exist but only in the same way the PC I am typing this on is an IBM PC derivative, not in the way people generally understand consoles as a thing.
A gamma ray burst wipes out all technology and probably a lot of life such that PS6 is the last thing on anybody's mind.
Someone cracking some kind of streaming service (the technology and infrastructure is already there, would cost a fair bit to implement though) such that the PS5 or something remains for a long time with a native library and streamed also library for the new shiny shiny is the most likely of those from where I sit, though I certainly hope the DVD player model is adopted.