I tried to move beyond archaic Java with managed VMs when they were first announced. Back then it was half-baked and glitchy, it felt like Google was using its actual users to do their testing, I hope it's gotten better since.

Generally, I think what Google is missing (or doesn't care about) is that anyone who is using a PAAS provider is willingly exchanging some flexibility, control, and price-efficiency for the luxury of having devopsy management tasks taken off their hands in a competent and reliable manner. Managed VMs seem to be providing so little of that that you might as well move to IAAS. Or, do as I have, and move to Azure - who do manage to stay pretty current, and blend PAAS and IAAS in a much more elegant manner.