As others have said, chasing old SPG LTP (maps to new LTP) is kind of crazy when you'd have to do a 50+ night mattress run in 2018 to achieve it. Since new LTP in merged MR will be possible with the same number of years (10 as plat) and only 100 more nights, it would make more sense to just take the next year or two to earn it.
If you keep or open MR Visa or SPG Amex, that will give you 15 elite nights per year to qualify in 2019 and beyond, and if MR Rewarding Events remains intact as-is (questionable), a sub-$100 meeting will garner 10 elite nights. This will make it easier to renew plat in future years and bump your night count towards lifetime. Of course, the Rewarding Events only makes sense if you have legitimate purpose for meetings, or if your stay activity is close (but not exceeding) the 50 nights to hit plat each year.