I believe there are posts at MRI often citing that MR had the only LT-status program among major hotel chains, but I guess this has all chagned now. LOOKOUT Marriott - others are following suit and surpassing your efforts. Qualifications are simple as pasted here:
Introducing SPG Lifetime Platinum! The best things in life – for life. Never go below Platinum membership level when you achieve 500 eligible nights total and any 10 years of Platinum SPG status.
My favorite item with SPG is their guaranteed suites on points (I just could not resist 
) :
Suite Night Awards (50 Eligible Nights). A Platinum Preferred Guest who has 50 Eligible Nights in a Qualifying Period is eligible to receive 10 Suite Night Awards. "Suite Night Awards" may be redeemed for advance confirmable upgrades for select, premium rooms or standard suites on a per room, per night basis, subject to availability.