Perhaps a fortuitous example, but carry that pattern out 2 more years - 350 is exactly the number needed in 1 year to accumulate 1000 nights for lifetime PLT without ever staying at a Marriott again (350+275+200+125+50), if carry-over continues.
I must say I'm surprised that carryover isn't just the number each year over 75 nights *in that year* as originally described. I have about 175 nughts to go and this feature will pull my qualification in by a year or so (and I have a long way to retirement). I hit both Silver and Gold this year (thanks to the reduced points level).
Rollover certainly makes earning lifetime status easier, now that nights is likely the limiting factor rather than points.