The house is the determinative for the word "Tomb". This may be because the tomb was considered to be the deceased's "house for eternity" i.e. the idea of living is in the background and what better way to express that by using a glyph that denotes living somewhere = house. That is what the determinatives are for!! Why would it be different for the pyramid-glyph? Why does it have to mean "pyramid" only? Obviously , evidence in the WB, the pyramid-glyph was used as determinative to write "tomb", a tomb that was not a pyramid! Now what?
An important clue, in my eyes at least, is the fact, that during the MK the afterlife of a commoner was beginning to look like that of a king, and hence the meaning of tomb may have changed somewhat i.e. instead of being the eternal dwelling, it may have become a place of resurrection for the commoner also. IMO.
This said, maybe we should just agree to disagree, eh?

Ritva