both, Power BI Designer and Excel will co-exist also in the future. There are some features that are only supported in Excel (e.g. Linked Tables) which will probably never make it to Power BI Designer but all features currently only available in PBI Designer will sooner or later make it into Excel.
one of the main ideas of PBI Designer was to make Power Tools accessible to all users even if they do not have access (or licenses/subscriptions) for the latest version of Excel.
At the moment PBI Designer is ahead of Excel in terms of features (bi-directional relationship, new DAX functions, ...) and also receives regular/monthly updates.