What is somewhat encouraging while reading the (grim) narratives of aviation accidents is this: how much has to go wrong for things to go seriously wrong (compare James Reason's Swiss Cheese model of accidents), and how most similar cases end well.

In this case, things that went wrong - and any one of them not going wrong might have averted the disaster:

1. Kids missed the original plane and had to get on this special charter flight instead.
2. Skyguide had just the previous year reduced staff from 3 to 2 controllers at night, and
2'. One of the controllers took an unofficial break.
3. There was an unexpected delayed A320 coming in, distracting the single controller.
4. The Tupolev was equipped with TCAS - if it hadn't been, TCAS on the other plane would not have issued an RA, and they might have stayed level or climbed instead of descending, as instructed by TCAS. (Though without TCAS it would not have been allowed to fly in Europe, probably.)
5. ATC happened to tell the Tupolev to descend, and TCAS happened to tell the 757 to descend.
6. That ATC instruction came exactly at the same time as the TCAS instruction on the 757, so the 757 crew didn't hear the ATC instruction to the Tupolev.
7. The Tupolev crew did not acknowledge that ATC instruction (which the 757 crew might have heard).
8. A bit later ATC repeated the instruction, and exactly at the same time TCAS on the 757 also repeated the instruction, so again they didn't hear the ATC instruction to the Tupolev.
9. Just then, the delayed A320 called ATC again, so the controller was distracted and did not monitor that his instruction to descend resolved the conflict successfully.
10. The copilot in the 757 had to go to the toilet just 18 seconds before all this happened.
11. It was night, making visual separation harder, possibly.
12. The 757 crew was trained to prioritise TCAS > ATC, the Tupolev crew was more or less trained to prioritise ATC > TCAS.
13. The Tupolev crew following ATC over TCAS received an ATC instruction that was in line with their right-of-way rules (left plane climbs, right plane descends), so it must have seemed more plausible.
14. The Tupolev crew had a first officer that suggested following TCAS instead, but he was not sitting in the RHS because there happened to be an extra Instructor Captain on that flight in the RHS who chose to follow ATC.
15. There happened to be a software upgrade at Zuerich ATC that night that disabled the Short-Term Conflict Alert light.
16. That software upgrade also disabled the phone lines, forcing the controller to waste time with the phone when trying to call Friedrichshafen for the delayed A320, and also prevented Karlsruhe ATC from warning Zuerich ATC of the problem.
17. ATC made a small, per se inconsequential, mistake of confusing "at your 2 o'clock" with "at your 10 o'clock".
18. The 757 crew waited 23 seconds to tell ATC that they were in a TCAS RA descent, by which time ATC was busy with the delayed A320.
19. The Tupolev crew was too busy/distracted to hear and understand that 757 TCAS RA descent call.
20. The ATC radar updated only every 12 seconds and did not yet reflect the 757's descent when ATC decided to instruct the Tupolev to descend.
21. The serious incident earlier in Japan with ATC/TCAS mismatch had not yet filtered through to all crews worldwide.

It's just so unlikely.