At the end of the Battle Phase, if this card battled: Activate this effect
Card games are all about reducing variance as much as possible
Using a transition sidedeck can you leave you vulnerable to alternate win condition decks like Burn or Empty Jar if neither your main or sidedeck has adequate coverage
Usually, if you are playing another archetype alongside Lady of Lament like Z-ARC or Dinomorphia, you would play the cards that go with their strategy like Supreme King Z-ARC, Supreme Celestial King Odd-Eyes Arc-Ray Dragon, and Crystal Clear Wing Synchro Dragon for Z-ARC Lady of Lament or Dinomorphia Kentregina, Dinomorphia Stealthbergia, and Dinomorphia Rexterm for Dinomorphia Lady of Lament
While it appears well get to enjoy the delights of the upkeep step a little while longer, in light of recent developments, we all may need to adjust to a world without upkeeps before long