The thing is, I'm not sure whether any of the benfits you listed here are unique to HH Diamond. Offhand, it looks to me like they're all also available to HH Gold, which requires only 16 stays.

There are a few additional perks of HH Diamond (like "Diamond Force", though if it only applies for paid stays now then depending on when you use paid vs award it may be a useless benefit to you), but those who pay for their own stays or are otherwise too budget conscious to ever pay for "Diamond Force" rates often feel that HH Gold is all you need (and HH Diamond is worth pursuing only if offered easy "Fast Track", which I've been offered in two or three separate years already!).

In fact that, rather than comparison to HH Diamond, is what makes SPG's lack of free breakfast (if you're not upgraded to a club level at a hotel which has breakfast in the club lounge) even for SPG Plat seem lackluster. For 16 stays I can get free breafkast everywhere with HH, but it doesn't matter how much I stay at SPG, I'll never ever get a free breakfast at 4P or Aloft... (And all my stays being on my own dime and on a tight budget, I have to choose 4P or Aloft over Sheraton most of the time.)

... However, in the interest of accuracy: HH has much higher thresholds for each elite level if you use nights, while SPG doesn't have a way to reach elite level in stays alone. So it's a bit hard to compare if you don't do all your stays a 1-night stays (in which case stay=night). For those who have mostly multi-night stays, it may actually take a bit fewer nights to reach SPG Plat than HH Gold, and way way fewer nights to reach SPG Plat than HH Diamond. (In my case, though, I'm a dedicated "hotel hopper", so stays=nights for me if that's what it takes.)