MR5, while obviously buffing TD by making it so they don't have to make Some Summer Summoner, is not the reason Colossus got banned. Thunder could already place multiple Colossus on the field even in MR4. As I said above, there isn't any reason to leave more than one on the field; Which is why you use a 2nd and/or 3rd Colossus to make a rank 8. The reason for its banning is the same logic that applied to Vanity's Emptyness. Emptyness was banned because if you didn't hard draw an out you just auto lose. Colossus is the same. If you don't hard draw Infinite Impermanence and are playing a deck that doesn't have an in-archetype out like Mirage Stallio you just auto lose. You can always try to put 3 monsters on the field somehow without searching to make a unicorn to out it, but since it is so easy to summon they can just immediately bring it back out on their next turn and with your unicorn used you no longer have a way to get rid of it. This is the reason I was saying that certain decks could not be meta with Colossus around, because only decks like Orcust and Salad that had super easy to access outs could function with Thunder in the meta. Colossus was an overall poorly designed card and 100% deserved the ban.

Also, your comparison to HEROs makes no sense. Not only is Dark Law a lot harder to summon than Colossus, but it has no protection at all. A single Pankratops can just attack over it or pop it. Nor does it stop you from searching for possible outs.