#4 He could legitimately injure himself
I say this as someone who's grown up watching The Undertaker in action, time and time again. He's a man who, while careful, lays it all on the line for his fans. Along the way, the man has sustained many an injury that would have felled a lesser man than him. However, The Undertaker is still going strong now.
I do hope that The Undertaker does not become a casualty like his comrade- Sting. While Sting seemed to be in great shape against Seth Rollins, a few buckle bombs knocked the wind out of his sails. They would eventually force the legend to retire.
You never know how far you can go, especially after a lifetime of wrestling. The Undertaker is a family man now and a legitimate injury would be bad news indeed.