They used to do fusions with existing rares, but they haven't for a long time now, unlikely that they'll do it again - and if they do, of course you could still get them from the events then.
What does happen, very occasionally, is that a certain older rare suddenly becomes relevant. For instance, for a long time nobody cared about Metalshaper - until the Scarab King boss in Doom Tower was released and suddenly any champion who could have perfect uptime on a shield became very valuable. Or now with the new fusion, the void epic Demytha is very interesting and allows you to build new unkillable Clan Boss teams - including one with the rare champ Heiress, who was also mostly useless until now.
Last but not least, the most likely use for those rares is in the Faction Wars, where you need whole teams from just one Faction - so if e.g. there is only one AOE decrease defense champ in the whole faction, like Fleshmonger for the Skinwalkers, then perhaps at some point you'll want to build him, when trying to finish the Skinwalker Faction War.
So it's not 100 percent useless to keep at least one copy of every non-farmable rare... but it is a lot of space just for the limited chance of them ever becoming important. Probably you could be selective and try to keep only champions who do something unique, or at least fill an important role within their own faction.
