Two more seasons in the bank is an eternity, during which he'll have had two more playoff runs, the team will have made other moves, players will opt to stay or move on, players will drop off or maintain their present clip, Samsonov will have played a year in the AHL and another year as the NHL backup.
Without knowing how those other facets play out, without that context, it's impossible to say what the future holds. For now, they just traded an excellent backup goaltender as a sign of the faith they have in their Cup-winning starting goaltender, who was excellent in the playoffs.
The league is littered with goaltenders who can put up great regular season numbers and crumble in the postseason. Holtby's proven. When they replace him with Samsonov, it won't be done lightly. You can always sign Holtby for five years and trade him one or two years into his new deal.