Excellent question, which I know every detail-focused brewer wonders about at some point. The reason we don't go up to alpha temperature right away and then drop down is that the beta enzyme denatures relatively very quickly above about 150°F (65.6°C).
You could look into the actual science on this but in my estimation it seems the majority of beta enzymes are gone after the first 30-40 minutes at a temperature of say 155°F (68°C).
On the other hand, alpha amylase enzyme is very stable and actually active in a very broad range from way down in the 130-168°F (55-75.5C). Therefore, if you want the benefits of the beta enzyme, all you really need to do is prolong the mash TIME at a temperature of anywhere from 140-150°F (60-66°C), which will speed up the beta activity without denaturing it too quickly. I find a good compromise at about 148-152°F(64.5-66.5°C), or at the nice round number of 150 F (65.5°C). This is where I mash nearly all my batches, typically aiming for 148-150°F(64.5-65.5°C), and for just 45 minutes typically or about 60 minutes maximum, unless I want very high attenuation then I will mash for 75-90 minutes. I think mash TIME is too often overlooked as a tool for the brewer. Temperature is important, if you want to understand where the beta and alpha amylase are denatured.
As long as you have the temperature in range for both to be very active at say 145-152°F(63-66.5°C) or thereabouts, then the primary variable I think is TIME, not temperature.