I like this debate... I switched over to Marriott Jan 1 of last year to start accruing points in case that becomes a factor, and I already had SPG LTP locked in.
Thankfully my travel only started with SPG in 2000, so if 15 years is the marker, I likely lost less than 50 nights worth of accrual points wise.
I am hoping my 630 SPG nights along with my 150 new Marriott nights carries the day, and I stay LTP.