Dang this stinks. I'd be ticked off too. I fully agree SPG should make this right. Clearly you are the type of customer this chain is looking for. A loyal customer who stays a substantial amount of nights per year. It would mean absolutely nothing to SPG to give you PLAT. And for those who say "rules are rules", try an get off your high horse. If the OP had exactly the number of stays he says he had, you can't really think he/she should not be allowed a tiny bit of leniency?
100 nights per year, even at a modest rate of $200/night is $20K in spend. OP may not have the best tact, but it sounds like he/she is the kind of customer I'd want to keep happy and in my hotels, especially as airbnb and similar hospitalities slowly chip away at the market share of these giant chains.
In a strange turn of events, I also recently lost my PLAT status (it was my fault). I politely sent them a Facebook message yesterday and this morning I woke up to a response saying they were upgrading me back to PLAT as a one time courtesy. May be worth a shot for you?
Good luck.