If I remember correctly, Hilton Diamond was created in the 80's as an invitation-only level in Honors above Gold. There were no published qualification levels for Diamond, Hilton contacted you if you traveled enough. Fast forward to today and all it takes is a credit card and you qualify. When Diamond for a credit card started, I was sure that Hilton was planning to create a new higher level, to maintain a way to reward truly loyal customers, but that never happened. In some ways, Lifetime Diamond now plays the role of the level above Diamond. Higher-end Hilton properties that cater to US-based travelers are swamped with Diamond guests, so in order to get a better room or a late check-out, it does help to be LTD.
The look back into the past sure is interesting. Lots has changed in credit cards and travel over the past 40 years. Hell, lounges already feel like part of a past life, to me. I would caution that those who set the baseline for what "used to be" to today's benefits may not be accurately depicting the actual value. For better or for worse. It could also be an indicator of what the future holds.
At the end of the day, for those looking for that way of truly being rewarded for their stays, there is no defining perk of being lifetime over diamond. Sure some properties may prioritize it. It will be personal preference if 10 years at the highest loyalty level and 1000 nights/2 million points is worth it for the possibility of chocolates or possible upgrade preference.