Consider this campaign where PCs can't get particular items back ever but hold false hope that they can. Now consider one where PCs can't get the items back until they beat the very last boss. There is no difference between these campaigns until the end of the last session where the PCs win and either do or don't get their items back.
And if retrieving the items isn't the main quest; if your goal is to have your PCs give up on getting the items back and have other adventures in a world that just no longer has these particular very rare magic items, that's... not really so big a deal. If missing these magic items isn't what's driving the plot, then your players will barely notice, especially as they gain levels and find other magic items.