A lot of it boils down to blood pressure. High blood pressure is a serious contributing factor to cardiovascular incidents (as well as a slew of other negative health risks), and getting a good night's sleep will help keep blood pressure down. This is also why the amount of heart attacks are up around 24% after daylight savings[1]; an hour less sleep means higher blood pressure means higher risk of heart attack (relative to any other 'normal' day).
I can definitely see how the same logic could apply to Mondays. Less sleep, more stress = higher blood pressure = higher risk of heart attacks.
[0] https://www.goodreads.com/book/show/34466963-why-we-sleep
I disagree. It's not that the symptoms aren't there, but that they have become normalized due to obesity, smoking, etc. being commonplace. Shortness of breath, sleep apnea, feeling weak, upper body tension/pain, etc. are usually present for quite a while in most people before it finally happens. People don't check their blood pressure often enough despite it being so cheap and easy to do.
It was once I lost 100lb that it became apparent just how bad condition I was in even though at the time I thought I felt fine. It doesn't help when you mention a lack of energy, or poor sleep to the doctor and they just say "Everyone is tired!".