I'm keeping a couple of my SPG cards open for a couple of reasons. First, I generally squeeze out close to $100 or more in Amex offers so most of my $95 Amex cards pay for themselves or more when adding offers to AU Cards.
From a long term perspective I'm wanting to hedge and see what happens to the accounts.
The accounts either get sold to chase so ultimately I'll end up with more CL to allocate Among chase cards (not that I need it ) or Amex will become a secondary issuer of Marriott cards like they are with Hilton alongside Citi. In that case maybe the features or earn structure are better than the Chase product. I have the Ritz card but don't use it for any spend but the $10k spend per year to maintain MR gold.
Finally, the accounts are backdated with Amex so I would always change to another no fee Amex if given the chance