Sony has trademarked PlayStation 6-10 in Japan. Now a simple answer for why they are doing this is for the future. They are securing name legally for future consoles down the road.
For those who don’t know if you have the trademark then you have the rights to the product. No one else can claim it unless they want to be sued for copy write among other crimes. Now since we are having the PS5 in 2020 it is obvious that we will be waiting for these things to come for awhile.
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