The biggest risk is that when the programs merge, maybe they say: SPG Lifetime Gold is grandfathered into Marriott Lifetime Gold, but going forward lifetime status will be earned under Marriott rules -- and Marriott current rules say that 250 lifetime nights (plus some other stuff) gets Lifetime Silver while 500 lifetime nights (plus some other stuff) gets Lifetime Gold. In that case, you'd be short of Lifetime Silver and would be a long way from Lifetime Gold.
So, that one particular scenario would kind of suck, and might be worth a bit of investment -- how much will vary by person, but I could imagine it might be worth $1k anyway? -- to lock in SPG lifetime gold status now. But even then I don't think it's worth anything like $10K, and Marriott may choose a different set of rules for how they treat lifetime status going forward that makes this hypothetical irrelevant anyway.