Iran doesn't want to sink merchant ships. They want to extort money from merchant shipping companies by threatening to sink their ships if they don't pay for 'protection'. All they need is a credible threat, which they already have absent any naval ships willing to stay at point blank range to defend merchant ships.

While there are religious, cultural and political aspects to this, the Iranian govt has primarily become a kleptocracy in recent years. It sustains power through the Revolutionary Guard (aka IRGC) which has grown into what's essentially a state-run, money-making commercial enterprise. It collaborates and colludes with various entities across the Iranian economy which it controls either directly or via bribes and coercion. While reasonable people can debate what the recent attacks on Iran accomplished, they certainly nerfed a large part of the IRGC's income. The new Hormuz extortion scheme isn't just retaliation or vengeance, it's replacing lost income which is urgently needed to prop up the Iranian government.