No they wouldn’t it would probably be on Unreal Engine 6 just how they switched tekken 7 from unreal 4 and tekken 8 is 5. They would never use old tech for a new game. Majority of Fighting games use new engines.More than an added tax, he's goes into depth about continued support of an online game being costly.
If they were to make a Tekken 9 for current gen, development will not be as high because everything is pretty much already done engine wise.