Sentry doesn't record anything.
It just moves the last 10 minutes of dashcam files from recent to saved when it goes to alert mode.
Sentry will work just fine with NO usb storage-still turn on, still go to alert mode with flashing lights and HAL 9000 message- it just won't move over anything since there's nothing to move.
Anyway, mathwise, 4 cameras are writing 7.2 GB per hour to the recent folder, that's 172.8 gigs per day... or 1.35 full write cycles on a 128GB drive.
The cheapest flash memory out there is rated for 1000 write cycles, the decent cheap stuff (which is what I believe Samsung uses), is rated for 3000... so that's just over 2 years to reach 1000 write cycles, or just over 6 years to hit 3000.
For most folks who are only running the cameras/sentry about 1/2 to 1/3rd of their day on decent cheap stuff you're talking 12-18 years to use up a 128GB stick.
Either way you shouldn't be anywhere near the limits of the HW today....and high endurance memory won't change that.