Complex sentence: They still remained very popular throughout the 90s, but began to decline at the turn of the milenium as glasses became easier to get and more affordable thay were fasionable for men and women and are still fashionable today but as retro sunglasses.
Split into simpler sentences: They still remained very popular throughout the 90s, but began to decline at the turn of the millennium as glasses became easier to get and more affordable. They were fashionable for men and women and are still fashionable today but as retro sunglasses.