Homework: Numbers

It's not impossible for a deck to have no number monsters. It's also not impossible for a deck to have no effects which destroy. Not a lot of ways for someone in that position to get over the number monsters are there?

Because there are some decks that rely on the deck itself and not extra decks. By having this number rule, cards like utopia would be indestructible and could save the player from what should be their defeat.
For example, my favourite deck relies on Yubel. This means that I need to be wise with my cards as I need to pay the tribute to summon and use Yubel, by using XYZ's or Synchros I would eliminate Yubel and it's usefulness from my deck.

More or less what the other's said. Number Wall is a card now so that's a thing mayne.

"Activate only if you control a "Number" monster. "Number" monsters on the field cannot be destroyed by card effects, and cannot be destroyed by battle except with another "Number" monster. When a "Number" monster you control is destroyed, destroy this card."

That said, making that a built in feature to every number I agree is insanely dumb. To specifically answer your question as to "Why remove this feature?" Well the answer is simple fairness. By making number some unkillable beat stick you would make it so that players who did not use number cards, or XYZ at all even would have to destroy the opposing monsters with unconventional means. Having destruction prevention built in as an effect is one thing. But when applied to an entire series of Extra deck cards the rule would have been far too trolly.

what we have to do?

The homework is rather unclear and the way you worded the question makes me think that you are asking for agreement. Although this post has already expired and does not matter, I am still confused and would like know how I could answer this in the future.

Slightly stated, many players wish to challenge themselves and by doing this limit certain cards in their deck and extra deck, say like......the numbers