Earlier quoted context omitted.
FWIW, this scenario is what many people jump towards in missing-persons cases, and it so far has not been true even once in my experience. It does happen, but it's much rarer than a straightforward combination of bad circumstances and bad decisions. Hell, at this point I've seen more murders and probable murders than people disappearing on purpose.
Likelihood is irrelevant -- what matters is if it was covered. For example, if he bought "How to Disappear Completely and Never Be Found" on Amazon, was doing Google Searches for it, etc. -- seems not only worth further investigation. Research shows that if for example during an interview his wife said it was possibly a suicide, that the odds are good it was: https://link.springer.com/chapter/10.1007/978-3-319-40199-…
No search operation has unlimited resources. If you had unlimited resources available, then sure, you can run down every imaginable possibility. But, real-world, it's not like that: you've got a few groups of people, maybe, who can devote some time to one case, and then they've gotta move on to the next one.
Your scenario is also essentially impossible to disprove. A detective might examine a home computer and find no evidence, but what if he used his phone? What if he used some other workstation? What if he knew someone? There are an infinite number of what-ifs, and striking each one off the list -- which is time-consuming and expensive -- only leads to another one.
There are few certainties in cases like this. All you get is the evidence and your ability to put the pieces together. After that, it's all probabilities.