but, as far as I know the last mid air collision involving a Tornado before this event was in 1990
The previous Tornado collision was with a Cessna in 1999. Having said that collision is not the risk, it a consequence of the risk not being adequately mitigated. In this case installation of CWS. The "Top Event", main risk or incident, is "Airprox". (Obtained from MAA Bowtie data).
A CWS would act as (1) "prevention" againt the Top Event (Airprox) occurring, and (2) "mitigation" in order to reduce the impact of of the Top Event should it happen. In TCAS terms TA and RA.
Not having an accident is no guarentee of safety. Nimrod flew for 30 years with fuel couplings in close proximity to hot air pipes. In this case there was neither prevention nor mitigation.
N.B For the avoidance of doubt Airprox means "Aircraft in close proximity to another aircraft that their safety is, or may be compromised"
DV