As an iPad Mini 5 owner, I don't see any reason to be ashamed to hold onto your current device.
As an iPad Mini 6 preorder, I can tell you regardless of this downclocking news it will still run rings around the 5th gen in terms of performance.... so there's no shame in upgrading either.
Do what's best for you and your wallet.
My cost to upgrade to the 256GB WiFi model from a 64GB WiFi iPad Mini 5 is $434 after trade-in. That satisfied my wallet and my perceived value in upgrading. It may not fit for others. I use the Mini as a dedicated mobile gaming device, also streaming via the AV adapter, and I've hit the 64GB storage limit already and had to offload titles. The list was too good to pass up.