https://www.dailymail.co.uk/health/article-8875201/Has-Covid-killed-flu.html
This article suggests that if a host body already has a virus, it is more difficult for a new virus to gain traction. (or at the very least, closely related viruses). e.g. Covid and flu.
If so, the presence of SARS-CoV-2, the virus that causes Covid-19 and has run rampant throughout the world, has somehow 'crowded out' the flu virus.
A Chinese study on two previous coronavirus outbreaks, SARS and MERS, has also shown the same effect. Infection with another virus, such as flu, protects to some degree against a coronavirus infection.
So the flu may have actually killed SARS and MERS as flu season came around.
The lockdowns and general social distancing, have no doubt had an effect on the transmission of the flu in 2020. Like Covid, the flu is also respiratory diseases that spread through similar means.
If pathogenic competition it is true, herd immunity could be built against many viruses using an artificial virus that has no negative impact.
The Oxford vaccine uses a dummy virus taken from a Chimpanzee that is inserted into the body.
Using this approach, antibodies for all the main known viruses could be triggered by dummy virus, creating immunity to many viruses and diseases just by blocking other "competitor" viruses gaining traction.
like Artificial intelligence. A man-made virus that can create anti-bodies for other viruses- or perhaps even block viruses from similar families taking hold.