Most of this is BS and straw man arguments that I've already either explained or debunked, and won't bother doing so a second time. I will, however answer your honest question:
Why do you believe character diversity is important?
What is the number one complaint about Smash4 right now? What was the number one complaint about Brawl?
It was/is that everyone was sick of seeing the same two or three characters constantly.
I never said character diversity is important. Melee did well enough with four characters for most of its life. It is, however, interesting. Interesting to play against. Interesting to watch. Every day there is a new thread or article filled with people wailing about "needing to rally support" or "how to deal with x character". A one character strike solves both.
It's simple. Suppose I'm sick of hoo hah, as many people are. For a while, people will probably default to banning Diddy when in doubt. What does this accomplish?
1) There is less incentive to have a pocket Diddy, making it easier to focus on characters you actually want to play.
2) Characters which are completely stopped by one or two hard counters suddenly become much more viable, again allowing you more freedom in choosing a character competitively.
3) Instantly, the game becomes more interesting to watch for a number of reasons, the least of which being that you don't have long sets of 5+ games involving the same two characters on Smashville.
4) The game becomes more enjoyable to play, unless you live and die by the hoo hah, or love fighting against it every other round.
I like Mega Man. Let's say my Mega Man is so good that people who play against me start banning him instead of Diddy. Then what? I never get to play Mega Man again?
Wrong.
I'll just use Rosalina, Mario, Luigi, Pac-Man, Lucario, or Bowser, all of whom I enjoy and play either well or competitively. How many times do you think people will lose to Rosalina before they decide to start taking their chances with Mega Man? You can make the argument that you only want to play as a certain character, and for beginners that is especially true, but I cannot think of any top-level competitors who do not have at least a secondary, if not thirds and beyond. Chances are that your character has at least one bad matchup, and most people will pick up a second to deal with it, even if they only want to play as that one character.
Right now, we counterpick characters and let matchups shape the meta game. It makes many characters completely unviable, places a ridiculously disproportionate amount of attention on the high tier characters with the fewest bad matchups. This solves the problems of the "snowball effect" of tiers, and of limited character viability without actually banning characters from play. It also encourages variety for the players, which is helpful to them (see Dabuz vs M2K) and more entertaining to watch since a greater breadth of skill is displayed. It also adds a strategic element to the match. Will you strike your opponent's main or your character's counter? Do you think your opponent will strike your main, or have they prepared a traditional counter and will strike your second?
Flexibility with this rule allows for many possibilities. For example, you could only strike one character only after losing a match (one character total for each player in a 3-game set). Alternatively, both players could strike after every game, or you could even make strikes last one round before the character becomes available again. It wouldn't take very long to find the sweet spot, and once we found it, the competitive scene would become vastly more interesting.