Of course it may provoke delays/problems/whatever, but data is still there.
If the sequence is:
1) encrypt user files
2) get admin access and crypt MBR (in such a way that is not decryptable)
the "destructive part" is #1, the MBR can be rebuilt from scratch just fine (and possibly if the reboot is somehow prevented the encryption key may be recovered from RAM as it happened in some cases for Wannacry, in which case it is #3 that creates more damage).
The doubt is when the "Kaspersky vendetta happens", if it goes:
2) get admin access and crypt MBR and 24 more sectors(in such a way that is not decryptable) OR check the presence of Kaspersky and write junk to first 10 sectors of disk
It doesn't anyway change anything, there is no real difference between rebuilding a MBR (or GPT) because it was crypted and rebuilding it because it was overwritten by junk data, but once you have rebuilt it, if the files are encrypted you have lost them all the same.