Learn English – Word for hardwork gone into drain

[Edited:] You couldn't say last morning in most cases, because you might be referring to this morning if the hour of speaking is late (today's morning is the "last" morning then): it would be confusing if last morning could be either today or yesterday. The same applies to the afternoon. But you can't go later than "night": the last night that has passed (you would not use "last" for a period that is not over yet) is necessarily that of the day before today. This could be the reason why we use "last" only with "night". But there might be some entirely different historical reason instead; it is impossible to say without textual research.

We could theoretically use yesterday night as an alternative to last night; but it would be longer, which might be why it is never used. Or it could be some other reason.

The typical word for the general phenomenon of employees leaving an organization is attrition.

This word in reality can have a more significant meaning in the sense of loss of intellectual assets, thus causing a reduction in the company's overall capabilities.

So, I think you can use attrition in its primary sense or its metaphor here.