We are only able to draw a probability cloud referenced to elapsed time. Then, as with any foam it begins to coalesce, or collapse as its tiny bubbles burst and gases carry away the aromas with them. Along with the degradation of the total brew due to escaping flavors you are also losing texture as the foam, or crema, dissipates. Texture is featured throughout the cuisine and I prefer silky, buttery textures to my espresso and cappuccino. The crema also protects the delicate aromatic flavors from acids which are also present in the brew.
All roasted coffee has a acidic compounds in it.
During brewing, as you make a liquid from the roasted coffee, acids are allowed to mix with flavor compounds, which they instantly attack and shatter the frail bonds holding these compounds together. To repeat, the acids if allowed to mix with coffee flavors alter the molecular structure of the flavor, breaking it down. This process never improves the flavor molecules, it only degrades them into what we detect as bitterness. So crema, which acts like a semi-solid protects aromatic flavors from acidic compounds by holding them suspended and away from the bully acids. Like a referee keeping the combatants apart.