The point is to dilute the pool from getting what you want so that you’ll continue to want it. That, and the fact that if you remove the chaff, there’s only wheat left — that breaks the economy.
Your gold keys, for instance—they have a small chance to drop gems. If the chance to drop troops decreased to 0 over time as you acquired them and ascended them to mythic then the rates of other resources (like gems) would increase commensurately (eventually to 100%). That makes them way more valuable—too valuable, in fact, because the average endgamer could probably open hundreds (if not thousands) of gold keys a day for a GIANT pile of the game’s most valuable currency for free.
Instead, you get souls. Because all extra troops are worth is souls. Which you probably also don’t need in this position — the game knows this, but intends for it to be so to avoid cratering its economy with an influx of useful currency that would create rapid inflation issues.
And if event keys worked like guild guardians, people would easily be able to get the legendaries and epics they’re missing, which is the same problem from a different angle (just cards vs. gems, both of which are arguably their own sorts of “currency” within the confines of the game). That screws up the rate of card acquisition, making it too fast, and making it so that people wouldn’t want to spend them anymore because there would no longer be any need to.
Best advice is: if your collection is complete, hoard your keys to absorb the impacts of new content in the future. If you don’t, you’re just buying souls you don’t need, which is by design.