So "theoretical", that MS invented UEFI "Secure Boot", to prevent exactly this attack vector on the PC's boot process.
I think that you would be surprised to hear that boot-sector infectors, as well as rootkits, have merged. They call them "bootkits", and they are on the rise.
Therefore, a "clean" (which wipes more than just the partition table), is important, and if you really want to be sure, a "clean all" will wipe the entire host-addressable LBA range with zeros.
On the rise? Really?
The security community has an affinity for the exotic. Which is understandable. Yes, bootkits exist. There are even some that make use of VT-x and act as a hypervisor (which was a great excuse for Sony to go around disabling VT-x with no way of turning it back on some years ago). But they are "theoretical" (okay, poor choice of wording: I should've said "effectively theoretical") because they are virtually never seen in the wild. And when they do appear, they are used in highly-targeted attacks against high-value targets. They make for great showpieces at security conferences and in research papers, but their real-world viability is... questionable. Not something that most people would have to worry about.
Anyway, I hope you are not forgetting that the boot sector is literally just that: a single 512-byte LBA sector. GPT extends this to about a few dozen sectors. We're talking about a region that's measured in kilobytes. Wiping the first "million sectors" puts you well past the MBR and into the actual partitions themselves, which in modern Windows starts at 1 MiB (it starts much earlier in NT 5.x and earlier--after just a single "track", which is defined as 63 512-byte sectors). Wiping the first million
bytes will destroy anything that exists before the partitions themselves (and let's throw in the final million bytes to wipe the backup table for GPT). There is absolutely no need to wipe anything beyond that, and certainly no need for a wipe of the entire LBA range. That's useful only for scrubbing data from a drive that you are discarding or selling to prevent someone from salvaging private data from it. But from a malware security standpoint, there is absolutely
no benefit whatsoever from something like that. Period.
In any case, I think we all agree that malware targeted at point-of-sale machines designed specifically to steal card data in-memory after a swipe is... not going to be seen anywhere outside of point-of-sale machines.
And our only disagreement is my assertion that there is no evidence that boot attacks are anything but rare and targeted. When I "format" a disk in preparation for a fresh new Windows install, I actually just use the Windows installer to delete all the partitions and let Windows recreate everything from scratch (the installer will automatically create a new MBR in this case and/or convert between MBR and GPT as needed). This is trivially simple to do and does not involve unnecessarily zeroing out an entire disk.