Yes, stopping at 1.5 is now impossible. But stopping at 2 isn't impossible and stopping at 3 is realistic. Stopping at 5 will be catastrophic. Let's not do that.
"Too late" is also a bit of an academic question as well; there are dragons lurking in the Earth systems - tipping points where carbon sinks become carbon sources and other feedback mechanisms waiting to be tipped. It's very easy to imagine that one tipping point cascades into another to create a runaway effect, though I don't believe there is scientific consensus on exactly where these points are, so we can perhaps hope to avoid them. That the oceans are so much hotter now than models predicted is an ominous sign that our understanding of such a complex system is still quite limited.
Isolating everyone to only ever see the place they were born and never travel the world seems like a big overreaction to me. Transport (and specifically aviation) is hardly the biggest offender as well. Energy, Industry, and Agriculture are 80% of emissions, we should start there.
But if I really wanted to start a flamewar, I'd suggest agricultural changes; that is eating less beef in particular and meat generally. There are reasons so many religions have diet restrictions - it's a fairly fundamental part of human identity.
These efforts targeted at changing individual behavior are distractions from actual things that economies could choose to do centrally to largely fix the problems. There is no reason we need to continue burn coal/gas to generate power, except that we think it is too expensive to make the effort to switch over. For example, Germany choosing to shut down nuclear power plants while there are still fossil power plants on their grid means they don't really believe that climate change is as big a threat to civilization as they say elsewhere.
It's one thing to do something to make yourself feel comfortable, it's another to say things to make others feel uncomfortable.
But life is often uncomfortable; sometimes you must endure a needle or a surgery in order to receive life-saving medicine. My city in South Texas hit a record heat index of 117 last year - I can assure you that that is very uncomfortable, and yet we are still in the relatively early stages of global warming; I assume that I'll need to move north at some point in my life.
But even I fear some discomforts - I cannot imagine a frank discussion with children today about the future they are staring down, and I have to imagine its one that nearly all parents will face (not that I am among them).
I may be wrong, but I think larger actions like replacing power plants, etc. are far easier to do at this point.
Both transport and heating each comprise more than 20%.
And telling people they shouldn't eat meat will likely result in even more backlash than telling them they shouldn't fly.
Nature doesn't care how much enjoyment people get from travelling the world. It just experiences the impact on the atmosphere.
Energy, industry and agriculture all improve everyone's lives, while less than 20% of the world population has taken even a single flight.
3 degrees basically requires countries to meet their commitments. Which for the developing world basically means net zero electricity generation by 2035, 99% of new vehicle sales being EV by 2030 and no new fossil heating in a similar time frame. That's 3/4 of the major emitters. Those are the "easy" ones, since those 3 changes all save money over the medium term. Some sort of technological or social miracle in the 4th (agriculture, especially meat) would give us a lot more slack.