Since you don't want monthly premiums (other than the Part B premium that you must pay, no matter what, if you want Medicare anything), that leaves you with a zero-premium MA plan as the only choice.
I suggest you use the Medicare.gov tool to look at MA plans available at your zipcode, filtering for Zero-Premium. Last year, I looked just to see. I live in an area with many hospital choices. The zero plan listed ONE hospital in my county in their plan... a hospital I never heard of. It may be an old RV parked behind a building. I think an HMO would be even drearier if your mandated PCP is a Gris-gris man. It was abysmal.
When people talk about having a "MA Plan", I really don't think they mean a "Zero-Premium MA Plan", at least not around here.
As to your question "is Original Medicare that bad?" (meaning having Medicare Part A & B, but no Medigap Plan, and no Part D)... well, it's better than what elderly people had before 1965, which was, nothing. Something was a lot lot better than nothing, but having to cover the 20% with the sky's the limit did start to perturb some, as did drug costs that skyrocketed over the years. The optional coverage enhancements have to cost somebody something.