Taken to extremes, it's easy to see how a virus that is 50% fatal within 24 hours will not last long itself.
Is there really very much selection pressure between 1% and 2% fatality rates over 4 week timespans, though? Especially if immunity is conveyed by infection, I don't see any reason why a virus like that would evolve to be less fatal within the timescales that humans care about.
Generally if it's twice as fatal it'll probably makes people twice as sick so they're more likely to go out less, stay home more, and less likely to infect other people.
However if it did damage in a way that doesn't manifest for a long time this wouldn't necessarily be the case. But that would be the exception more than the rule.
Is there really very much selection pressure between 1% and 2% fatality rates over 4 week timespans, though? Especially if immunity is conveyed by infection, I don't see any reason why a virus like that would evolve to be less fatal within the timescales that humans care about.