Players develop and improve at different rates, you can't hold on to a player for 5 years in the 'hope' he will continually improve.
I didn't think Moult was anything special, I thought Archer was dreadful. that was based on their time with us so much influences a player's quality, it's quite likely that our coaching was nothing special so moving elsewhere with a better coaching set up could be one of many reasons these players (and many others) have improved since they left us.
And it is easier to be better with better players around you...