It's impossible for them to support a 10-bit 6k@120Hz monitor with current hardware and keeping the old one around would be embarrassing. The Pro 5k will probably sell better/be more profitable anyway.
Apple refuses to remove things without clear replacements, for better or more often worse.
The 6k XDR has been replaced, apparently they've got something coming for the Mac Pro. I don't know why it wasn't the Mac Studio update last year. Maybe we find out at WWDC this year.
How does a 5k display replace a 6k display? Are they giving up on 6k? Disappointing.