That's what we did in high school back in the 90's. Nothing live having five "black lotuses" in your deck.

They can. My friend got his whole deck proxied online for about $50

Problem is that it’s worthless to anyone but him.

The cards are expensive but they also hold some relative value (can be resold) and only real cards are legal in tournaments.

That said, for casual games, proxies are growing in popularity and I always say they’re a moral imperative. Only problem is some people use proxies that use non-standard art or borders, which can really hamper the game (or in one case, be incredibly cringe and awful to play against).