The estimates I've heard of percent of users who use Reddit via third party apps are 5-10%, so nowhere close to a majority. I wouldn't be surprised if for power users the fraction is much larger though.

As a user of RIF I will be very sad when they shut-down. I'm hoping there's some alternative platform I can spend time on, but it's hard to see myself fully leaving Reddit given the sheer utility of it from an information stand-point.