"Startups ruin you" can't be rewritten as "It ruin you," though, yeah? It could be rewritten as "They ruin you," however. "Startups" is plural. "A startup"

"Startups ruin you" can't be rewritten as "It ruin you," though, yeah? It could be rewritten as "They ruin you," however. "Startups" is plural. "A startup" or "that startup" and so on is singular. "A startup ruins you" would be a fine English sentence.