NOT a historian here, but:

Roman numerals are awful: you can’t easily do basic arithmetic with them

Yes you can. Roman numerals seem a lot harder than decimals because you've spent your whole life around decimals rather than Roman numerals, and have literally decades of experience with decimals rather than roman numerals.

, you can’t do fractions with them

I/V is a fifth with Roman numerals. You absolutely can do fractions with them. They're the same numbers, just represented differently.

by definition you can’t do decimals with them.

1 + 1/10 + 1/100 +1/1000 = 1.111

I + I/X + I/C + I I/M = the same thing.

But why do you need decimals? If you need more precision, you can use a smaller unit - instead of talking about 0.01 dollars, you talk about 1 cent.

and the number zero wasn’t a concept yet.

The number wasn't, but the concept of nothing as used in a numerical system (often by leaving a section blank, or (first by Babylonians IIRC) writing a _ to indicate that the section/numeral is deliberately blank) had been around for ages.

with such a sucky number system.

If it was really that sucky and fundamentally limiting/flawed, then the Romans couldn't have built beautiful buildings like the coliseum, or developed such an enormously compex society. But they did, so perhaps it's not as bad as you assume?