Era was a collective term.
Undertaker was the final wrestler to have wrestled in all monikered Eras of wrestling - Golden Era (80s - mid 90s), Attitude Era (mid-very early 2000s), the Ruthless Aggression Era (early-late 2000s) and the fan-named PG Era (current).
Not only has his career spanned over 4 eras, but his wrestling career to WWE (as Undertaker) has lasted over 20 years, including 20 appearances at WrestleMania - more than anybody - and over 27 years of wrestling for mainstream companies.
Triple H came in right before the Attitude Era started; around 93 or 94. He was a prominent role in the Attitude Era, and even more so in the Ruthless Aggression Era.
Shawn Michaels is in the same boat as Taker (although working for WWE for longer).
The Era that Ended was not any single Era, but an Era of inception. Those who helped to build the WWE and also shape it to its current product are about to retire or already have.
Think of all the gimmick matches introduced by Undertaker. Think about how unrealistic his gimmick is, an undead 'zombie' type of mortician, yet how popular it is. How he's opened the doors for gimmick possibilities, albeit WWE Creative failing on the most part.
Think about how Shawn Michaels had his controversial Heel turn that would be still be relevant and even replicated in 20 years (by Jericho {ironically, Jeri-Clone is more appropriate here}).
Yes, Undertaker and Shawn didn't put wrestling on the map. Nor did they maintain a typical 'hero', but they introduced something so much more.
The End of an Era, as I said, was not an individual Era but everything those those 3 wrestlers represented, introduced, allowed or gave prestige to.
What's next after Undertaker, the only wrestler who truly defines a 'gimmick', retires? There won't be another. What about all his gimmick matches? Buried Alive was one of the most badass matches there are. Gone since 2010, so far. The only match that will remain is the Ambulance Match (what the "Last Ride Match" was based on).
No more Casket Match.
No more Buried Alive Match.
Hell in a Cell will remain, but its reputation was formed mostly by Undertaker, Triple H and Mankind (2/3 of whom are still with WWE), followed by Shawn Michaels and Kane (primarily because of his debut).