In some places the reason I don't use a GC is because I'm lazy and there wasn't one sat there ready to go. It is possible that I should learn to use the Boehm GC in these cases and that I should evangelise its use.
But in a lot of places it's because it's because the code's on the critical path. I don't see a GC (and particularly one you admit is "computationally expensive") buying us more than it costs in these codepaths. This is optimised code, where people have chosen the algorithms very carefully and read the latest papers, and run benchmarks to find good parameters and done all that good stuff. And we already have (by the nature of the system) an insatiable appetite for RAM so we certainly don't want to make that any worse.
Perhaps a better goal is to stop using C at all for the "lazy" code. I probably spend as little as 30% of my programming time writing C these days anyway, maybe it should be less than 10%. Java for example seems to work well enough, it's slow and it wastes a lot of RAM, and it's missing a bunch of features I like, but it works well enough. Probably a lot of my "lazy" code could be Java, more than already is.