Many people, myself included, have expressed how 3d Sonic games seem to lack precision. It somewhat worked on previous 3d Sonic games due to their design. If you are going to add complexity to the level design as shown in this trailer and screens, changes need to be made to accommodate them. Seems like a great choice.I wouldn't be surprised that the entire tiered speed system was created because the game likely doesn't have gravity implemented in an easy to understand way and/or the system isn't very friendly to human error. Needing not one but two buttons to reach top speed (which is likely required to navigate certain sections of the stages) is needlessly complicated. The use of single button isn't even necessary (see Sonic Adventure and Sonic Adventure 2). They most likely had players careening off the stages entirely during gameplay tests so they decided on a tiered speed system. That's probably the same reason why the parkour system exists, too.
Nice thinly veiled insult. How mature of you.
Plus the article states they went with that different direction from the conceptual phase, so I doubt the changes where made after starting playtesting, but rather where implemented first and designed around. .