JS numbers speculatively using integers in practice rely on the carry flag for cheap overflow bailout checks. Maybe if the overflow check didn't have the same problem, they'd look at that instead, but I don't see why they should have to switch if it doesn't gain them anything.
And there's tons of reasons why using one lane of vector units as scalar ops is an issue. Power, lack of ubiquitous vector units, their pipeline is commonly decoupled from the integer unit making branches goofy, etc.
