The saints would see the kingdom coming and LIVE THROUGH the "world ending" arrival and would recognize what had happened

Commentary On Mark (TNIGC) R.T. France [Mk 9:1]

The perfect tense of εληλυθυiaν indicates that they are not to see the 'coming' of the βασιλεία but rather to witness the fact that it HAS come. The prediction thus focuses not on its arrival, but on the point at which its presence, already a reality, is (a) visible and (b) displayed ἐν δύναμις toν θεόῦ (pg 344)

[The New International Greek Commentary, The Gospel of Mark, Editors I. Howard Marshall and Donald A. Hagner, 2002 Wm. B Eerdmans Publishing Co. Grand Rapids, Michigan 49503]