I think searching a landfill is a last resort: what a waste of time if it turns out a victim was removed from the building by other means.
We have no idea where the forensic evidence was found, when it was collected, how long it took and how many experts it took, to analyse it and confirm the DNA was Heidi's, and that it indicated homicide.
Then, IMO, there is collecting and analysing all the other evidence, including hundreds of hours of footage from cameras, to identify a time, place of death, and potential suspects, and interview them and others, to try to trace exactly the movements of individuals who may have interfered with the remains, and where they may have taken them.
IMO, homicide detectives don't have the luxury of dreaming up theories, they have to collect actual, physical evidence, in an exacting, precise manner, not just to get search warrants, and proceed with further investigation, but in order to actually convict anyone of crimes.
JMO
Don't know about LAST resort (maybe 'most likely' place they think she is) but the 'by other means' gets overlooked by many. Why would the 'vanishing act' actors wait for the next routine (normal) pickup? Seems they might want her as far away as possible, as soon as possible.