Well, I mean that there is already a part 6, the Jolion series with Jolyne Kujo. But I believe what you mean is that, Will There Be a Anime to Part 6. The answer will be yes. They have followed the inital story arc, Phantom Blood to Golden Wind. They need to make a Part 6 anime due to its importance to the whole story arc. Now, Will There be a Part 7 and 8 anime?, Probaly not.
Probably the person thinks like that because nothing's listed anywhere, it was the same with golden wind, there were already series announced ahead for 2017 but no golden wind till 2018, it was put later. Just because it isn't announced yet doesn't mean it won't happen sooner...2021 though isn't unlikely.