The emphasis is on ἀποθνήσκοντες dying. The Levites are dying men, who pass away in due course, and are succeeded by others.
But there (ἐκεῖ δὲ)
In the case of Melchisedec.
(He receiveth them of whom) it is witnessed that he liveth (μαρτυρούμενος ὅτι ζῇ)
The Greek is very condensed: being attested that he liveth. The A.V. fills it out correctly. Melchisedec does not appear in Scripture as one who dies, and whose office passes to another. See on abideth continually, Heb 7:3.