"Giving the immune system practice" doesn't apply to covid. Every single covid infection makes the immune system weaker.
Your suggestion is akin to saying condoms are unnecessary, everyone should just get HIV and we should just accept the new normal.
There's no easy way out of this pandemic, just like there was no easy way out of the HIV pandemic.
Everyone’s health situation is different, but I think comparing this to HIV is a bit of a stretch.
I do wish everyone has the choice to decide what works for themselves. In my case, I do just that and I don’t subject myself to anything I don’t want.
Being bedridden for 1-2 weeks after getting the flu is also very uncommon since the fever lasts 3-4 days on average. Here's a graph of viral load over a typical 5.5 day infection: https://royalsocietypublishing.org/cms/asset/3a62f972-1633-4...
It's very common if you have a high number of international participants. In Munich, the most nasty occurrence has its own name: "Wiesngrippe", named after the Oktoberfest which serves as a living petri dish with millions of people, but it's also common after the huge trade fairs (e.g. Bauma).
Service staff are hit hardest because many of them don't take off days if they're sick - they power through for as long as they can (because they can make above 15.000€ in the two weeks!) and end up spreading all the germs around.
In addition to that, SARS-CoV-2 is suspected to be able to induce immune amnesia similar to what can be observed after measles - anecdotally, I've been incredibly sensitive towards anything from the common cold to outright influenza ever since I caught COVID last year. It's no fun to travel anywhere when you know you'll catch some bug because yet another moron decided to not at least wear a mask or stay home when sick.
[1] https://www.abendzeitung-muenchen.de/muenchen/das-verdient-e...
Especially considering how with every previous congress the congress flu had infected the vast majority of participants and caused them to be bedridden for the 1-2 weeks after.