From the experts I've been listening to, true. Masking and distancing has prevented colds and flu. Also, not having kids in school and not going to work. I also would guess more of us got the flu shot -- I got my first one ever. If masking does go away and distancing does go away, and that isn't going to happen any time soon, I imagine that yes, these diseases will come back.
I had Covid and it drags on forever but was not scary for me...but Influenza is excruciatingly painful (and RSV was not painful but is the only virus I went to an ER fearing death.) I fear Influenza so much I have been doing masks and Listerine for years but all it takes is that one exposure (RSV) during a lapse in following my own advice. I am curious whether Influenza could be eradicated in the future.
At our house, we haven't had one cold this whole last year, and it's almost like our state didn't even have a flu season due to the flu shot and wearing masks and all. I don't think we'll have a worse cold and flu season if Covid is defeated and we stop wearing masks, I think it will just revert to the past levels of colds and flu. If we have to mask indefinitely for Covid because of new variants, it will certainly help the world not get a lot of colds and flu. But unless for some reason there is 100% compliance the world around for masking, distancing and washing the hands, unfortunately they'll be sure to come back.