However, most of the other arguments you provide rest on the premise that "the demand for low skill human labor will drop very close to zero as most menial jobs and quite many more complex jobs can be automated." This assertion is not supported by historical precedent: every time something has been created to reduce labor, we just find other ways to keep people busy
For the first time in hisotry we're starting to see a point in which machines are not just there to ease production in the hands of humans, but to take it over completely. Take something like drivers as an example: logistics is a huge part of modern day life in any economy, and moving stuff from place A to place B provides work for a lot of people. Now then, as technology has advanced less and less people have been able to transport larger and larger amounts of stuff. In the very near future we will start needing no drivers at all, as cars will navigate themselves.
So then, you say that these people who used to drive trucs and cars will simply do something else... what? By the time self-driving cars become common place, a good deal of other low skill jobs will have already gone. The number of warehousing jobs and data entry office jobs is falling as automated warehousing and scanning systems are takjing jobs away from both etc...
Of course technology creates some new jobs with it, but the point is that these technologies create less jobs than they automate. A completely automated warehouse will require maybe a handful of people to supervise and maintain the system wherein it used to employ tens if not hundreds of people etc...
You cannot simply assume work will pop up from out of nowhere for people, since most jobs uneducated people could do can soon be handled more cheaply and more efficiently by machines.
Where will they get their sense of achievement (assuming they're not very religious)?
Hobbies, arts, etc. I mean, we don't yet know, but we do know that many people simply will not have a skillset that will be worth enough in the market for them to be employed. It doesn't matter if you're a top notch welder once we have robots that can do pinpoint accuracy welding 24/7 with less mistakes than a human. That means people will have to start finding their sense of achievement in things other than work for the most part.
When living on UBI is comfortable, we've lost the "significant improvement" incentive
Well living on the UBI is supposed to be comfortable because as I'm trying to explain to you, it's pretty much inevitable at this point that within the next century most non-highly educated people will have to be without jobs. The alternatives are even worse: not having a UBI means these people will still be without a job and they can easily become a destabilizing force in the societies.
Being poor is not a purely economic problem. It's chiefly a social problem and yes, throwing money at the poor won't fix the social problem.
But it is also an economic problem and what you're not understanding is that you cannot fix poverty with incentive-based systems in a future in which the market has no need for the majority of people who have no valuable skills.
Slavery is morally wrong... and so is freeloading.
In a future in which there will only be jobs for a small segment of the populace 'freeloading' (ie. living unemployed) will be the norm, not the exception, and as such it cannot be seen as morally wrong. The idea that one has to expend X amount of physical/mental labor to be able to live 'morally' within a society can only be valid so as long as that is something that is possible for everybody to do, since that won't be the case very soon, saying that somehow because we've managed to use technology to reduce the need of workers (which is, in the end, the whole point of technology from the start) makes not working immoral is just nonsensical.
Even if you were right, these things aren't in the present. You're arguing that we will need UBI at some point, but use that as justification for it's creation now. How can you justify fixing a problem that doesn't even yet exist?
Because we can see the trajectory already: it will take time to get there, but we're in motion. Western societies are already seeing jobs disappear in large numbers due to technology and we can only expect this to keep going at an increasing speed. We have currently about 300 000 more unemployed people in finland than we have open jobs because heavy industry no longer employs anywhere near the amount of people it used to, and as I said earlier office jos are shrinking as well as automated electronic invoicing and other such systems reduce the need for office workers etc. It doesn't take a crystal ball to see where this kind of development is heading.
This is not something that's hundreds of years into the future kind of stuff, this is happening and this is happening fast, so we need to start getting prepared for it and start thinking about models to fix it such as UBI.