I'll agree with you that coverage hasn't improved much. But "Sprint has just stalled" is definitely not true. They've been working hard on upgrading and improving what they've got and it shows in the speed test data.
Has anybody that's not on the beta received the January security update? My Sprint S9+ is on the December 1st update and my AT&T S9+ is on the November 1st update. Both say they are up to date. I wonder if they're waiting on the official Android 10 release now instead of testing and approving more Pie updates.