Also, I wonder if what they used is a phase change compound instead, and the heatsink is subject to temperatures where the stuff stays molten. Or there's something about heat cycling the stuff that causes it to expand and ooze out cyclically.
They could just as well opt for a thermal epoxy, bonding it once and forever without any worry of shifting. An electrically insulating compound can be selected, and glass beads can be added to ensure consistent application thickness -- and thus breakdown voltage.
Service might be more annoying of course, but by gluing to a heat spreader, replacement is just as easy, or even easier perhaps.
Heh, for that matter, the kind of component-level repair possible on an assembly like this, need not be all that much. If a couple transistors die with arc flash, exposed connections on the board will be eroded. You can clean up the soot and the burn marks, but it's not going to be a brand new board.
Tim