Does untreated ADHD in adulthood lead to no real long-term consequences?

No, and this is a genuinely important myth to correct. Untreated ADHD in adulthood is associated with meaningfully worse average outcomes across several domains: higher rates of job instability, relationship strain, financial difficulty, accidental injury, and coexisting mental health conditions like anxiety and depression, compared with treated ADHD. References: https://www.caddra.ca/wp-content/uploads/Canadian-ADHD-Practice-Guidelines-4.1-January-6-2021.pdf