it was good enough in seasons 1 2and 3
It really wasn't though. Under that system of itemization, we had two major problems. First, carries couldn't viably itemize into defensive options. The opportunity cost was simply too high, with the sole exception of periods where GA was broken enough that 3-5 members of a team would purchase it. Having defense was not a real option, no more than buying an IE is for a tank trying to increase their damage nowadays. Doing it was a very, very good way to lose. It was always better to take the risk on your enemies making a mistake and you getting to be useful, rather than building a pure defensive item and guaranteeing you wouldn't be useful. Having items with damage and small amounts of defense (PD rework, Death's Dance) allowed for defensive itemization to actually be an option for carries, instead of just a trap.
Second, ADC itemization at the time was incredibly cookie-cutter. You had no choices except highest-dps, so that's what you did. It was the exact same items, in the exact same order, on almost every character. Adding items with alternative strengths allowed for giving up some dps to actually be a realistic option.
when adcs dropped building defensive items mages followed suit
Mages never bought defensive items. Mages who bought defensive items were either trolling or terrible, often both (again, excepting the "GA is OP" period). AD carries tried to in season 1 and eventually learned that it was a trap, but mage never were never stupid enough to believe that building a real tank item was anything but a trap. They built Zhonya's and relied on it if they needed safety.