Like many past great champions, there's always a player(s) who eventually will mature and step up to stop or at least slow down the current reigning champion. Borg's obstacle was McEnroe who snapped his 5 straight Wimbledon titles and denied him the USO twice. Sampras was denied 2 USO by the youngsters(Hewitt & Safin), and Federer snapped his chance of tying Borg's 5 straight Wimbledon. Federer also denied Agassi 2 USO. Nadal have prevented Federer from winning a few FOs, Wimbledon and AO. Nole right now is heading in the same direction as the former champions. I think Thiem and/or Zverev will be the next stars to repeat the same pattern by stopping Nole from winning more slams in the future.