Being the most popular mobile OS is not an easy task, especially when every OEM builds its own version of the Android OS, and unfortunately not always using the latest version to do that, either, which could lead to some security issues. Usually most of these vulnerabilities are discovered in "labs" (meaning they just researched them and discovered a new vector of attack, rather than finding some malware in the wild), while other times, most of the people who get malware on Android are either from China or Russia, where most app piracy exists (meaning they got it from 3rd party piracy websites, not from the Play Store).
Either way, Google will keep working to increase the security of the Android OS, especially since they want it to be the OS of choice for enterprise customers, too. Chrome OS has been received pretty well by enterprise customers so far, and in part because right now it's the most secure OS by far.
Part of that security comes from the "verified boot", which means the OS is verified block by block when it's booted, to see if there have been any modifications to its core or to files that weren't supposed to be modified by the user or a 3rd party. Here's exactly how it's supposed to work:
"The dm-verity feature lets you look at a block device, the underlying storage layer of the file system, and determine if it matches its expected configuration."
"...if rooting software compromises the system before the kernel comes up, it will retain that access. To mitigate this risk, most manufacturers verify the kernel using a key burned into the device. That key is not changeable once the device leaves the factory."
This sort of security could also make rooting a lot harder. After all, rooting is done by finding vulnerabilities in the security of the system, and then changing certain configurations through that vulnerability. You could see why Google would want to stop that. It's not just that good ol' ROM modders that could do this, but very nefarious people (or certain agencies), too. So I do think Google has not only the right, but the obligation to secure Android as much as possible.
This is all fine so far, however, the problem comes when certain OEM's will enable this feature in the future in their ROMs, and will not allow the users to unlock the bootloaders if they want to. Securing the device is fine - but if a user wants full control over his bought and paid for hardware, then OEM's should absolutely allow them that power, and they shouldn't lock the devices unless they provide that unlocking capability, too.
The second problem is that even if some OEM's allow you to unlock the devices, carriers will force them to keep the devices sold on their network, locked. Carriers like Verizon especially, have always loved to exercise this (undeserved, possibly illegal) power of theirs, which should probably be overseen by FCC.
While some sites are trying to blame Google for this, Google so far has made Nexus phones that are very easy to unlock, and HTC and Sony also tend to allow their customers to unlock the phones, in general. But if you see a company that locks the bootloader and doesn't allow you to unlock it, then you should definitely speak up. It's the only way to keep our devices open, while also having them be very secure.