I would say, jolla inserted buttons here and there cause sailfish was selling very very (and i mean very) poorly and they hoped to sell more if people shuldn’t have to stare at an empty screen without nothing to clic/press/tap
People are used to the “in your face” android approach, rather than the “discover hidden things for a cleaner look” sailfish approach and that’s why they changed from sf 1 to 2, and then inserted buttons with 3 and 4…
Still, i would love to have sf 1 back, much more than any sf 5/6/7/etcetcetc