First of all, the Colossus is indestructible, so unless someone has a way to exile him, you have a huge 10/10 blocker. Then, if you can gather up 10 mana at once, he gains Monstrosity and 10 +1/+1 counters. Then, not only can attack as if he didn't have defender, but he gains trample as well.
Basically, you just need a ton of mana to make this guy work. Commander decks led by Ezuri, Renegade Leader (with all of the Green mana created by its Elf mana producers) to Animar, Soul of Elements (who can cast it for free) to Mayael the Anima (who can play it directly to the field with her ability). Creatures with unique combinations of abilities like this, no matter how expensive it may appear, will always have long-term appeal. Artifact creatures are also becoming more and more popular with more artifact support being printed all the time.
Do you see Colossus of Akros being worked into any Commander deck that you might have? If you don't, just keep in mind that trading it in to a vendor or local game store at bulk rare value isn't the best way to unload your Colossi.
So not only was the Colossus a Limited bomb and a competitive Constructed no-show He's actually worth keeping around for the long-term.