how do we replace Obamacare?

I am going to admit my ignorance and just ask this question aloud, why are we against just allowing anyone who doesn’t have insurance to buy it from Medicare? A lot of young people like me buying insurance from medicare would be premium dollars that would help support the program without much expense since I don’t go to the doctor very often at all. Thats how all insurance works. Healthy people subsidize the sick (thats how life insurance works, except you aren’t sick, you’re dead).

I mean instead of a 20,000 page bill for Obamacare we just need a one line bill, anyone who wants to buy insurance from Medicare can get coverage.

I fully realize this is simplistic but I am asking for why it woul;dn’t work so I can understand the issue better. I really don’t know anything about health care, and I am not even up to speed on teh basic reasons we need to stop Obamacare too. So if anyone can share their knowledge with me that would be much appreciated.