Definately an excuse. The Sunderland Airshow had been in decline for a number of years before the universal covid/climate change excuse. During the 2019 airshow, a BAC Strikemaster burst at tyre on landing at Newcastle airport which caused a number of diversions on a peak weekend at the end of July
In terms of aviation interest maybe (like most airshows) but attendances were fine. Not sure of the relevance of the Strikemaster to the cancellation?
The Triathlon World Championships eh?
Between January and October 2019 staff of the ITU took 1,471 flights covering 4,181,200 km and producing approximately 790 tonnes of CO2.
The Triathlon World Series events have roughly 120 athletes competing from all over the world, all of which will need to travel to Sunderland along with coaching and technical staff, family, friends and other associated hangers on.
I would suggest that the "environmental impact" of a small airshow mostly attended by locals will produce orders of magnitude less CO2 than an international sporting event.
It's the spectators attending the event which have the carbon impact. Isn't the Tour de France the sporting event with the largest carbon footprint?