Personally I don't see older cards having much more to rise, unless somehow mining becomes significantly more profitable (i.e. crypto value increasing a lot faster than mining difficulty and electricity costs).
I guess it depends a bit what utility people are currently getting from their cards, e.g. are they mining themselves or using the GPU regularly in a second PC or whatever. If it's literally sat around doing nothing I would sell.
It's a tough one, on the one hand right now I don't think my son would notice much difference if I sold the RX480 and put an old 7950 in (he's too young to play hardcore games, Fortnite etc), on the other hand in a couple of years as he gets more used to PC gaming he might, and I'd hate to be in a situation where it would still cost several hundred pounds to get RX480 performance levels back. Also provides me a backup card if my 1070ti were to die for some reason.