Should I worry about WordPress security at MLM scale?

Yes, and the stakes are higher than for typical content sites because distributor PII plus financial data raises the threat profile. Production MLMs on WordPress need: managed hosting at the Kinsta or WP Engine class (shared hosting is not viable for production MLM), a security plugin like Wordfence or equivalent, a WAF in front (Cloudflare's pro tier or similar), 2FA on all admin and distributor accounts, weekly backups verified by restore test (a backup that hasn't been restored is a hope, not a backup), and prompt patching. The investment is roughly $200 to $500 per month all-in for the security stack; this is a real line item, not optional.