Yes - you get paid more the higher you're drafted.The difference between 2nd round and 3rd round is a lot of money.
The average 2nd round pick earns $2M per year for four years including the signing bonus.
The average 3rd round pick earns $1.375M per year for four years including the signing bonus.
The average 4th round pick earns $1.13M per year for four years including the signing bonus.
Watch the full draft & count how many times when a Senior gets drafted you hear an Analyst say "I liked his Junior/Sophomore tape better"
Your argument/opinion only considers the most positive outcome for Taylor/Kinchens/Williams if they returned for their Senior Year.
Progression is not linear - you don't just automatically get better every year you play. Losing money by coming back is also a potential outcome.