The fundamental problem is that you cannot realistically social distance on a flight or through an airport. Screening does not work: they screened people prior to letting them onto repatriation flights but many of the passengers went down with the virus within days of landing. Mass testing is not available, time consuming and not particularly reliable.
People coming off a flight are travelling for a reason, be it work or pleasure. And that almost always involves high levels of social interaction. So you may get on a flight with it, you may catch it on the flight, if you have it you are almost certain to spread it after you land.
I'm in the oil industry: we ask people to quarantine before they go offshore, and screen them before they do. Some companies test: we don't, because on all the evidence the tests available in the UK are nowhere near reliable enough to make a material difference. We don't allow people offshore if they are high risk (pre-existing medical condition). There is COVID on many offshore installations, including three of ours. And there have been quite a few medivacs of very sick people who were not viewed as high risk.
So, absent a reliable test, absent a vaccine, asking people to quarantine after a flight is not unreasonable. A lot of people will not travel because they won't want to quarantine, either at the destination or on the return. Even without quarantine, they will likely choose not to travel because they view the risk of catching the virus whilst travelling, and having to deal with it away from home, as a risk they do not need or want to take.
This may be a deeply unpopular view if you work in the travel industry. But it will likely be the reality for 12 - 18 months.