Not the GP, but it looks like significant number of Android vulnerabilities [1] are local privilege escalation (as in: install an app from a questionable source, app use vulnerability to silently gain root, app siphons off your data and/or wrecks havoc on the phone).
Hefty fraction of local privilege escalation vulnerabilities are in the Linux kernel, and pretty much every single Android device out there ships with its own, unique, outdated out-of-tree kernel with dependencies on a number of version-specific binary-only blobs. End result of all that is - even if underlying vulnerability is fixed in mainline kernel, phone's vendor still needs to repeat this exercise for each and every phone. Moreover, because of binary blobs, phone vendor oftentimes has to contact their SoC vendor, SoC vendor has to contact the vendor for that IP block they've shipped binary blob for, etc... All this process is extremely fragile, costly, and lengthy. Not exactly something you want to have in place for security updates. Google somehow manages to do half-decent job updating their Nexus/Pixel phones for few years after hardware hitting the market. Most other vendors drop the ball to some degree here, often completely.
Replicant took on a Sisyphean task of enabling blob-free (a.k.a. 100% free and open-source) firmware to some Android devices. Among other things, this could eventually enable one to use mainline kernel and do security updates much more quickly and efficiently.
Current status however is far from that ideal. Latest "stable" Replicant (4.2) uses various versions of 5-year-old kernel 3.0.x branch [2,3], and didn't achieve fully blob-free status either. That said, you can run blob-free, as long as you can do without 3D, WiFi, bluetooth, hardware-accelerated video and (depending on the hardware) without camera, GPS and NFC too [4].
[1] https://www.cvedetails.com/vulnerability-list/vendor_id-1224...
[2] https://git.replicant.us/replicant/kernel_samsung_smdk4412/b...
[3] https://git.replicant.us/replicant/kernel_samsung_espresso10...
[4] http://redmine.replicant.us/projects/replicant/wiki/Replican...