“I see nothing special in their return. Wagner is no longer a powerful force. Those who returned are far from being in a good fighting mood, as they know what to expect here,” Cherevatyi said. “Furthermore, they are now under the control of the Defense Ministry.
That's pretty important. It seems like Wagner had an effective command structure that allowed for successful offensive capabilities. The Russian military will just mindlessly march them into Ukraine's meatgrinder.
The plane was shot down over Russian airspace with a radar jamming missile.
Also, the top three members of Wagner were all on the plane, and there was no clear line of succession. Just a standing offer from Putin to roll the mercenaries into the regular army.
The other responses are good, but I'll just add that if it was Ukraine, they would take credit for it. Assassinating a high ranking officer is a big deal, and would make them look strong. So unless there's something else going on, it's safe to rule Ukraine out.