Keys are always rising. Unless Valve does something to make refined metal more desirable, keys will always rise and will probably never fall. If anything, keeping ref would only lower your backpack value as time goes on.

TL;DR, just constantly resell.

Better margins in item flipping. Keys should stop rising soon. If they continue to rise much higher (as in 8 ref) I would be concerned.

Keys will never stop rising, unless the game collapses, and then its all for naught.

Keys are linked to money, metal is linked to free, so of course it rises. Everyones fighting over the same few keys just a tiny % of users buy and actually trade on instead of uncrating. That facts not going to change anytime soon. That being said metal is useful, you can't buy things without it unless you are a high tier item spender only. Key rise has slowed over time, so if you lose a scrap in value every couple weeks in refined, its basically nothing, you can't buy things to use without it. Since you only have 2 keys i doubt you buy every item in the key range....