Helvetica is weak in small sizes

It is said that Helvetica is not suited for texts in small sizes. When the system fonts for Mac OS X Yosemite are changed to Helvetica, many designers claimed that Helvetica is not appropriate one.

“Helvetica sucks” by Erik Spiekermann

You can see the low legibility of Helvetica if you type texts in a small size and make them blur. Some texts become blended and hard to decipher. They say that Apple developed San Francisco fonts in order to make small size texts in Apple Watch more legible.

The letters become blended in small size texts

But nowadays, smart devices have more resolution than paper printing, and texts in iPhone are not always as small as in Apple Watch. Why Apple changed the system fonts for iOS and Mac OS X, not only for Apple Watch?